MIPS: pte_special()/pte_mkspecial() support

Add support for pte_special() & pte_mkspecial(), replacing our previous
stubs with functional implementations.

Signed-off-by: Dmitry Korotin <dkorotin@wavecomp.com>
[paul.burton@mips.com:
  - Fix for CONFIG_PHYS_ADDR_T_64BIT && CONFIG_CPU_MIPS32.
  - Rewrite commit message.]
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org