MIPS: mipsregs.h: Add read const Cop0 macros

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: James 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/