MIPS: Add helpers for assembler macro instructions

Implement a parse_r assembler macro in asm/mipsregs.h to parse a
register in $n form, and a few C macros for defining assembler macro
instructions. These can be used to more transparently support older
binutils versions which don't support for example the msa, virt, xpa, or
crc instructions.

In particular they overcome the difficulty of turning a register name in
$n form into an instruction encoding suitable for giving to .word /
.hword, which is particularly problematic when needed from inline
assembly where the compiler is responsible for register allocation.
Traditionally this had required the use of $at and an extra MOV
instruction, but for CRC instructions with multiple GP register operands
that approach becomes more difficult.

Three assembler macro creation helpers are added:

 - _ASM_MACRO_0(OP, ENC)
   This is to define an assembler macro for an instruction which has no
   operands, for example the VZ TLBGR instruction.

 - _ASM_MACRO_2R(OP, R1, R2, ENC)
   This is to define an assembler macro for an instruction which has 2
   register operands, for example the CFCMSA instruction.

 - _ASM_MACRO_3R(OP, R1, R2, R3, ENC)
   This is to define an assembler macro for an instruction which has 3
   register operands, for example the crc32 instructions.

 - _ASM_MACRO_2R_1S(OP, R1, R2, SEL3, ENC)
   This is to define an assembler macro for a Cop0 move instruction,
   with 2 register operands and an optional register select operand
   which defaults to 0, for example the VZ MFGC0 instruction.

Suggested-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Marcin Nowakowski <marcin.nowakowski@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17770/