MIPS: MSA: Update helpers to use new asm macros

Update MSA control register access helpers to use the new helpers for
parsing register names and creating custom assembly macro instructions.

This allows the move via $at to be dropped (saving a total of about 20
bytes of kernel code).

Note, this does not alter the equivalent code in .S files, which still
uses the $at trick.

Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17776/