MLK-14768 fec: Change CONFIG_ETHPRIME name to align with DM ETH

MLK-14768 fec: Change CONFIG_ETHPRIME name to align with DM ETH

The DM Ethernet driver requests the "ethprime" to align with DTB name
or start with "eth" with seq number as index.

So previous name "FEC" can't work as prime. Must change it to the "eth0" for
first ethernet device, or "eth1" for second one.

Signed-off-by: Ye Li <ye.li@nxp.com>