[MIPS] Fixe the definition of PTRS_PER_PGD

When we use > 4KB's page size the original definition is not consistent
with PGDIR_SIZE. For exeample, if we use 16KB page size the PGDIR_SHIFT is
(14-2) + 14 = 26, PGDIR_SIZE is 2^26，so the PTRS_PER_PGD should be:

	2^32/2^26 = 2^6

but the original definition of PTRS_PER_PGD is 4096 (PGDIR_ORDER = 0).

So, this definition needs to be consistent with the PGDIR_SIZE.

And the new definition is consistent with the PGD init in pagetable_init().

Signed-off-by: Dajie Tan <jiankemeng@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>