rdfremont: Set MHUv3 memory addresses

Define some possible MHUv3 connections and devices in rdfremont and
set the addresses of all the Mailbox (MBX) and PostBox (PBX) in the
LCP.
These addresses will be used by the MHUv3 driver to perform
MHUv3 operations like triggering a message transfer.

Change-Id: I106fcf6889877cc9e8d096ab34764d7b8b148e69
Signed-off-by: Tomás Agustín González Orlando <tomasagustin.gonzalezorlando@arm.com>