net: pch_gbe: Allow build on MIPS_GENERIC kernel

MIPS Boston board, which is using MIPS_GENERIC kernel is using
EG20T PCH and thus need this driver.

Dependency of PCH_GBE, PTP_1588_CLOCK_PCH is also fixed for
MIPS_GENERIC.

Note that CONFIG_PCH_GBE is selected in arch/mips/configs/generic/
board-boston.config for a while, some how it's never wired up
in Kconfig.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230607055953.34110-1-jiaxun.yang@flygoat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>