board: phytec: imx8mm: Add PHYTEC phyCORE-i.MX8MM support

Add support PHYTEC phyCORE-i.MX8MM SOM.

Supported features:
 - 2GB LPDDR4 RAM
 - 1x 1Gbit Ethernet
 - eMMC
 - external SD
 - debug UART3
 - watchdog
 - i2c eeprom

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>