MIPS: allow msa.h to be included in assembly files

Just #ifdef away the C functions when included from an assembly file,
as will be done in a following commit.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7299/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>