MIPS: Cosmetic cleanups of page table headers.

MIPS: Cosmetic cleanups of page table headers.

   * Clean up white spaces and tabs.
   * Remove _PAGE_R4KBUG which is no longer used.
   * Get rid of hardcoded values and calculate shifts and
     masks where possible.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8457/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>