arm64: dts: imx8mn-var-som: add SOM EEPROM

The 4Kbit EEPROM located on the SOM contains hardware configuration
options, manufacturing infos and ethernet MAC address.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>