MIPS: Don't assume 64-bit FP registers for FP regset

When we want to access 64-bit FP register values we can only treat
consecutive registers as being consecutive in memory when the width of
an FP register equals 64 bits. This assumption will not remain true once
MSA support is introduced, so provide a code path which copies each 64
bit FP register value in turn when the width of an FP register differs
from 64 bits.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6427/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>