mips: zero out pg_data_t when it's allocated

This patch is preparation for the next patch which removes the zeroing of
the pg_data_t in core MM.  All archs except MIPS already do this.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>