[MIPS] Clean up asm-mips/mach-generic/spaces.h

PAGE_OFFSET definition is now using CAC_BASE by default.

This patch also reorder some macros to make them appear
in the same order for both 32 and 64 bits configs.

It also makes use of const.h generic header file to
annotate constants.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>