Some Cop0 registers are constant and have no side effects when read.
There is no need for the inline asm to read these to be marked
__volatile__, and doing so prevents them from being removed by the
compiler.

Add a few new accessor macros to handle these registers more efficiently
(especially for the sake of running in a guest where redundant access to
the register may trap to the hypervisor):
  __read_const_32bit_c0_register()
  __read_const_64bit_c0_register()
  __read_const_ulong_c0_register()
Signed-off-by: NJames Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Maciej W. Rozycki <macro@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17922/