nxp: Finish migration of SYS_FSL_SRDS_[12] to Kconfig

As this is used on both ARM and PowerPC platforms, this needs to be
listed in arch/Kconfig.nxp and match how they're currently used by
select'ing them under the required PowerPC ARCH_xxx options.

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