[MIPS] Sparsemem fixes

1. MIPS should select SPARSEMEM_STATIC since allocating bootmem in
   memory_present() will corrupt bootmap area.
2. pfn_valid() for SPARSEMEM is defined in linux/mmzone.h

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>