board: phytec: phycore-imx93: Set CONFIG_ETHPRIME to eth0

board: phytec: phycore-imx93: Set CONFIG_ETHPRIME to eth0

Set ethprime to eth0 since FEC interface is considered the primary IF on
phyCORE-i.MX93 SoM based boards. This comes from the fact that the same
bootloader is reused for both carrier boards, that is phyBOARD-Segin and
phyBOARD-Nash which both use different Ethernet PHYs on the EQOS (eth1)
interface and thus eth1 cannot be used as a prime.

Signed-off-by: Primoz Fiser <primoz.fiser@norik.com>