KVM: MIPS: Add callback to check extension

KVM: MIPS: Add callback to check extension

Add an implementation callback for checking presence of KVM extensions.
This allows implementation specific extensions to be provided without
ifdefs in mips.c.

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