net: Tighten more driver dependencies

net: Tighten more driver dependencies

In this case, the mediatek network drivers cannot build outside of
ARCH_MEDIATEK or ARCH_MTMIPS, and so express this requirement in Kconfig
as well. In the case of DWC_ETH_XGMAC / DWC_ETH_XGMAC_SOCFPGA, the file
controlled by the DWC_ETH_XGMAC option references a socfpga-specific
array defined in the file controlled by DWC_ETH_XGMAC_SOCFPGA. Rework
these options in Kconfig to handle this dependency.

Signed-off-by: Tom Rini <trini@konsulko.com>