boards: Eth Click MikroElectronica shield

Adds support for a new SHIELD, Eth Click.
Mikro-BUS type shield supported in Zephyr.
Adds Kconfig for mikroe_eth_click shield.

Signed-off-by: Andrei Gansari <andrei.gansari@nxp.com>

boards/shields/mikroe_eth_click/Kconfig.shield

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+# Copyright (c) 2020 Linaro Limited.
+# SPDX-License-Identifier: Apache-2.0
+
+config SHIELD_MIKROE_ETH_CLICK
+	def_bool $(shields_list_contains,mikroe_eth_click)

boards/shields/mikroe_eth_click/doc/index.rst

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+.. _mikroe_eth_click:
+
+Overview
+********
+
+ETH Click is an accessory board in mikroBus™ form factor. It features `ENC28J60`_,
+a 28-pin, 10BASE-T stand alone Ethernet Controller with an on-board MAC & PHY,
+8K Bytes of Buffer RAM and SPI serial interface.
+More information at `Eth Click Shield website`_.
+
+Pins Assignment of the Eth Click Shield
+=======================================
+
++-----------------------+---------------------------------------------+
+| Shield Connector Pin  | Function                                    |
++=======================+=============================================+
+| RST#                  | Ethernet Controller's Reset                 |
++-----------------------+---------------------------------------------+
+| CS#                   | SPI's Chip Select                           |
++-----------------------+---------------------------------------------+
+| SCK                   | SPI's ClocK                                 |
++-----------------------+---------------------------------------------+
+| SDO                   | SPI's Slave Data Output  (MISO)             |
++-----------------------+---------------------------------------------+
+| SDI                   | SPI's Slave Data Input   (MISO)             |
++-----------------------+---------------------------------------------+
+| INT                   | Ethernet Controller's Interrupt Output      |
++-----------------------+---------------------------------------------+
+
+
+Requirements
+************
+
+This shield can only be used with a board which provides a configuration
+for Mikro-BUS connectors and defines node aliases for SPI and GPIO interfaces
+(see :ref:`shields` for more details).
+
+Programming
+***********
+
+Set ``-DSHIELD=mikroe_eth_click`` when you invoke ``west build``. For example:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/net/dhcp_client
+   :board: lpcxpresso55s69
+   :shield: mikroe_eth_click
+   :goals: build
+
+References
+**********
+
+.. target-notes::
+
+.. _Eth Click Shield website:
+   https://www.mikroe.com/eth-click
+
+.. _ENC28J60:
+   https://www.microchip.com/wwwproducts/en/en022889

boards/shields/mikroe_eth_click/mikroe_eth_click.conf

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+# ENC28J60 is L2 chip slave on SPI
+CONFIG_SPI=y
+CONFIG_NET_L2_ETHERNET=y
+
+# ENC28J60 Ethernet Device
+CONFIG_ETH_ENC28J60=y
+CONFIG_ETH_ENC28J60_0=y

boards/shields/mikroe_eth_click/mikroe_eth_click.overlay

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+/* Copyright (c) 2020 Linaro Limited.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&mikrobus_spi {
+	status = "okay";
+	cs-gpios = <&mikrobus_header 2 0>; /* CS */
+
+	eth_click@0 {
+		compatible = "microchip,enc28j60";
+		local-mac-address = [00 00 00 01 02 03];
+		/* Errata B7/1 specifies min 8Mhz, 20MHz max according to RM */
+		spi-max-frequency = <10000000>;
+		int-gpios = <&mikrobus_header 7 GPIO_ACTIVE_LOW>;	/* INT */
+		label = "ETH_0";
+		reg = <0>;
+	};
+};