board: ti: am57xx: Set ethernet MAC addresses from EEPROM to env

The MAC addresses for the PRU Ethernet ports will be available in the
board EEPROM as an address range. Populate those MAC addresses (if valid)
into the u-boot environment so that they can be passed on to the
device tree during fdt_fixup_ethernet().

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>