eth: mcux: Enable ETH_MCUX_0 conditionally
eth: mcux: Enable ETH_MCUX_0 conditionally

ETH_MCUX_0 needs to be turned on when CONFIG_NET_L2_ETHERNET is enabled.
If CONFIG_NET_L2_ETHERNET is disabled (e.g. when configured for 6loble),
then ETH_MCUX_0 should remain disabled.

Signed-off-by: Vakul Garg <f516ce29719b5820d33bb10d114ab6dad5fab601@nxp.com>

ldts/zephyr,zephyriot/zephyr,mbolivar/zephyr,mbolivar/zephyr,mbolivar/zephyr,galak/zephyr,galak/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,punitvara/zephyr,zephyriot/zephyr,zephyriot/zephyr,explora26/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,explora26/zephyr,galak/zephyr,kraj/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,finikorg/zephyr,mbolivar/zephyr,zephyriot/zephyr,ldts/zephyr,kraj/zephyr,punitvara/zephyr,punitvara/zephyr