MIPS: Use memblock_add_node() in early_parse_mem() under CONFIG_NUMA

Use memblock_add_node to add new memblock region within a NUMA node
in early_parse_mem() under CONFIG_NUMA, otherwise the mem parameter
can not work well.

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