virtio-mmio: Add explicit include for of.h

With linux/acpi.h no longer implicitly including of.h, add an explicit
include of of.h to fix the following error:

drivers/virtio/virtio_mmio.c:492:13: error: implicit declaration of function 'of_property_read_bool'; did you mean 'fwnode_property_read_bool'? [-Werror=implicit-function-declaration]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>