mips/kvm: Support MSA in MIPS KVM guests

Support the new KVM_CAP_MIPS_MSA capability, which allows MIPS SIMD
Architecture (MSA) to be exposed to the KVM guest.

The capability is enabled if the guest core has MSA according to its
Config3 register. Various config bits are now writeable so that KVM is
aware of the configuration (Config3.MSAP) and so that QEMU can
save/restore the guest modifiable bits (Config5.MSAEn). The MSACSR/MSAIR
registers and the MSA vector registers are now saved/restored. Since the
FP registers are a subset of the vector registers, they are omitted if
the guest has MSA.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
