MIPS: Declare various variables & functions static

MIPS: Declare various variables & functions static

We currently have various variables & functions which are only used
within a single translation unit, but which we don't declare static.
This causes various sparse warnings of the form:

  arch/mips/kernel/mips-r2-to-r6-emul.c:49:1: warning: symbol
    'mipsr2emustats' was not declared. Should it be static?

  arch/mips/kernel/unaligned.c:1381:11: warning: symbol 'reg16to32st'
    was not declared. Should it be static?

  arch/mips/mm/mmap.c:146:15: warning: symbol 'arch_mmap_rnd' was not
    declared. Should it be static?

Fix these & others by declaring various affected variables & functions
static, avoiding the sparse warnings & redundant symbols.

[ralf@linux-mips.org: Add Marcin's build fix.]

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: trivial@kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17176/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>