drivers: i2c: tca954x: Add build assert avoiding prio issues

In order for TCA954X driver to work well, we know that
mux root must be initialized before channels.
(see #37786).

This commit is:
- Ensuring that this condition is met at build.
- Document needed values for menuconfig to help user.
- Edit these values for TCA954X test sample

Fixes #40833

Signed-off-by: Aymeric Aillet <aymeric.aillet@iot.bzh>

drivers/i2c/Kconfig.tca954x

@@ -13,9 +13,13 @@ if I2C_TCA954X
 config I2C_TCA954X_ROOT_INIT_PRIO
 	int "TCA954x root driver init priority"
 	default I2C_INIT_PRIORITY
+	help
+	  Should be lower than `I2C_TCA954X_CHANNEL_INIT_PRIO`
 
 config I2C_TCA954X_CHANNEL_INIT_PRIO
 	int "TCA954x channel driver init priority"
 	default I2C_INIT_PRIORITY
+	help
+	  Should be higher than `I2C_TCA954X_ROOT_INIT_PRIO`
 
 endif

drivers/i2c/i2c_tca954x.c

@@ -156,6 +156,9 @@ const struct i2c_driver_api tca954x_api_funcs = {
 	.transfer = tca954x_transfer,
 };
 
+BUILD_ASSERT(CONFIG_I2C_TCA954X_CHANNEL_INIT_PRIO > CONFIG_I2C_TCA954X_ROOT_INIT_PRIO,
+	     "I2C multiplexer channels must be initialized after their root");
+
 #define TCA954x_CHILD_DEFINE(node_id, n)				    \
 	static const struct tca954x_channel_config			    \
 		tca##n##a_down_config_##node_id = {			    \

tests/drivers/i2c/i2c_tca954x/prj.conf

@@ -1,3 +1,5 @@
 CONFIG_ZTEST=y
 CONFIG_ZTEST_NEW_API=y
 CONFIG_I2C=y
+CONFIG_I2C_TCA954X_ROOT_INIT_PRIO=61
+CONFIG_I2C_TCA954X_CHANNEL_INIT_PRIO=62