trusty-virtio: add support for 64-bit phys addresses for vrings

Work around limitation of vring descriptor structure that only
supports 32 bits addresses. On archs with 64 bit physical addresses
store top 32 bits of such address in currently unused reserved field.

Change-Id: I30333f6b8966db89bd4b8a699a72b80c6257a31f
Signed-off-by: Michael Ryleev <gmar@google.com>