module/mhu3: Add support for fast channels

This patch adds support for the Fast Channel Extension to
the MHUv3 drivers.
As the drivers have to be modified to use the transport layer,
this patch also includes changes to the doorbell channel usage
via the transport layer.

Change-Id: I4f48338baba4f2847bd4633b3d026933a64845d1
Signed-off-by: Tomás Agustín González Orlando <tomasagustin.gonzalezorlando@arm.com>