trusty: add trusty virtio driver

Trusty virtio driver is responsible for management an
interaction with virtio devices exposed by Trusty.
During initialization, this driver makes an smc
call to retrieve Trusty virtio device descriptor from
secure side, parses it then instantiates and configures
the specified set of virtio devices.

Change-Id: I20a23518d5c19bedd9282f7f1d4b16957ba75408
Signed-off-by: Michael Ryleev <gmar@google.com>