KVM: MIPS/Emulate: Implement 64-bit MMIO emulation

Implement additional MMIO emulation for MIPS64, including 64-bit
loads/stores, and 32-bit unsigned loads. These are only exposed on
64-bit VZ hosts.

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