imx8mn-var-som: adjust PHY reset gpios according to hardware configuration

For SOM with the EC configuration, the ethernet PHY is located on the
SOM itself, and connected to the CPU ethernet controller. It has a
reset line controlled via GPIO1_IO9. In this configuration, the PHY
located on the carrier board is not connected to anything and is
therefore not used.

For SOM without EC configuration, the ethernet PHY on the carrier
board is connected to the CPU ethernet controller. It has a reset line
controlled via the GPIO expander PCA9534_IO5.

The hardware configuration (EC) is determined at runtime by
reading from the SOM EEPROM.

To support both hardware configurations (EC and non-EC), adjust/fix
the PHY reset gpios according to the hardware configuration
read at runtime from the SOM EEPROM. This adjustement is done in
U-Boot (OF_BOARD_FIXUP) and kernel (OF_BOARD_SETUP) device trees.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>