board/rdfremont: Define MHUv3 devices and driver interface

Define and initialize all MHUv3 device structures as supported by
RD-Fremont platform. Provide necessary APIs to interface with MHUv3
driver wrapper.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Change-Id: If273894dc0d926bdd6a407dc57790b2b80fdefbe