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/