doc: peripherals: add documentation for SDHC peripheral

add documentation for SDHC peripheral driver.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>

doc/hardware/peripherals/index.rst

@@ -35,6 +35,7 @@ Peripherals
    regulators.rst
    reset.rst
    rtc.rst
+   sdhc.rst
    sensor.rst
    spi.rst
    tcpc.rst

doc/hardware/peripherals/sdhc.rst

@@ -0,0 +1,49 @@
+.. _sdhc_api:
+
+
+SDHC
+####
+
+The SDHC api offers a generic interface for interacting with an SD host
+controller device. It is used by the SD subsystem, and is not intended to be
+directly used by the application
+
+Basic Operation
+***************
+
+SD Host Controller
+==================
+
+An SD host controller is a device capable of sending SD commands to an attached
+SD card. These commands can be sent using the native SD protocol, or over SPI.
+the SDHC api is designed to provide a generic way to send commands to and
+interact with attached SD devices.
+
+Requests
+========
+
+The core of the SDHC api is the :c:func:`sdhc_request` api. Requests contain a
+:c:struct:`sdhc_command` command structure, and an optional
+:c:struct:`sdhc_data` data structure. The caller may check the return code,
+or the ``response`` field of the SD command structure to determine if the
+SDHC request succeeded. The data structure allows the caller to specify a
+number of blocks to transfer, and a buffer location to read or write them from.
+Whether the provided buffer is used for sending or reading data depends on the
+command opcode provided.
+
+Host Controller I/O
+===================
+
+The :c:func:`sdhc_set_io` api allows the user to change I/O settings of the SD
+host controller, such as clock frequency, I/O voltage, and card power. Not all
+controllers will support applying all I/O settings. For example, SPI mode
+controllers typically cannot toggle power to the SD card.
+
+Related configuration options:
+
+* :kconfig:option:`CONFIG_SDHC`
+
+API Reference
+*************
+
+.. doxygengroup:: sdhc_interface