dev-arm: Add a MMIO transport interface for VirtIO

The MMIO interface currently only supports a subset of version 0.9.5
of the VirtIO specification. It has the following known limitations:

* The queue size hint (the QUEUE_NUM register) is ignored.

* Queue alignment is assumed to be hard-coded to
VirtQueue::ALIGN_SIZE (4096 bytes).

* Only 4096 byte pages are currently supported.

Change-Id: Ifd318f5e5bddab0b6a42d8c8af9ff2fbb477f98b
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Rekai Gonzalez Alberquilla <rekai.gonzalezalberquilla@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2326
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>