MIPS: add CONFIG_VIRTUALIZATION for virtio support

Add CONFIG_VIRTUALIZATION to the MIPS architecture and include the
the virtio code there. Used to enable the virtio drivers under QEMU.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2002/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>