[MIPS] Cleanup ARCH_DISCONTIGMEM_ENABLE and NUMA configuration.

IP27 configuration isn't the only NUMA system - it just happens to be
the currently only supported MIPS NUMA system.  So move the necessary
options back into the main MIPS Kconfig file.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>