KVM: MIPS: Claim KVM_CAP_READONLY_MEM support

Now that load/store faults due to read only memory regions are treated
as MMIO accesses it is safe to claim support for read only memory
regions (KVM_CAP_READONLY_MEM).

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org