MIPS: mark MSA experimental

MIPS: mark MSA experimental

In light of the commit 16f77de82f2d (Revert "MIPS: Save/restore MSA
context around signals") the MSA support in the kernel is incomplete.
Until the replacement for the former sigcontext changes is agreed upon
and in tree, mark MSA experimental & disable it by default.

MSA is only implemented by one CPU supported by the kernel, the P5600.
The P5600 is a 32 bit core, and thus MSA can only be used when the
experimental CONFIG_MIPS_O32_FP64_SUPPORT option is enabled. Therefore
MSA is only being used in experimental settings anyway and this change
doesn't actually make any difference beyond clarifying the state of
MSA support.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7311/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>