MIPS: asmmacro: Ensure 64-bit FP registers are used with MSA

This silences warnings like the following one when building with the
latest binutils:

arch/mips/kernel/genex.S: Assembler messages:
arch/mips/kernel/genex.S:438: Warning: the `msa' extension requires 64-bit FPRs

[ralf@linux-mips.org: Markos says binutils 2.25 and some 2.24 snapshots
are affected.]

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9745/


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