MIPS: Select ARCH_KEEP_MEMBLOCK if DEBUG_KERNEL to enable sysfs memblock debug

In the current code, CONFIG_ARCH_KEEP_MEMBLOCK is not set for MIPS arch,
memblock_discard() will discard memory and reserved arrays if they were
allocated, select ARCH_KEEP_MEMBLOCK if DEBUG_KERNEL to give a chance to
track "memory" and "reserved" memblocks after early boot, with this patch,
we can see the following two sysfs interfaces under DEBUG_FS.

/sys/kernel/debug/memblock/memory
/sys/kernel/debug/memblock/reserved

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>