clock_imx8mm: Add a stub for imx8mp_fec_interface_init()

clock_imx8mm: Add a stub for imx8mp_fec_interface_init()

When CONFIG_DWC_ETH_QOS_IMX=y and CONFIG_FEC_MXC is not selected, the
following warning is seen:

arch/arm/mach-imx/imx8m/clock_imx8mm.c: In function 'board_interface_eth_init':
arch/arm/mach-imx/imx8m/clock_imx8mm.c:914:24: warning: implicit declaration of function 'imx8mp_fec_interface_init; did you mean 'imx8mp_eqos_interface_init'? [-Wimplicit-function-declaration]

Fix it by adding a stub for imx8mp_fec_interface_init() to handle this
case.

This follows the same approach done on imx8mp_eqos_interface_init().

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>