trusty-virtio: Fix trusty_virtio_notify prototype

Current implementation of vring_new_virtqueue takes notify callback
that should return bool instead of void as it used to be. Update
an implementation of trusty_virtio_notify to return true to indicate
that virtqueue is alive and well.

Change-Id: I8ce06317a86ab5c4943023f048d7fb1d9e956d68
Signed-off-by: Michael Ryleev <gmar@google.com>