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>

arch/mips/Kconfig

@@ -2115,10 +2115,9 @@ config CPU_MICROMIPS
 	  microMIPS ISA
 
 config CPU_HAS_MSA
-	bool "Support for the MIPS SIMD Architecture"
+	bool "Support for the MIPS SIMD Architecture (EXPERIMENTAL)"
 	depends on CPU_SUPPORTS_MSA
 	depends on 64BIT || MIPS_O32_FP64_SUPPORT
-	default y
 	help
 	  MIPS SIMD Architecture (MSA) introduces 128 bit wide vector registers
 	  and a set of SIMD instructions to operate on them. When this option