StandaloneMmPkg: Fix support for receiving MM requests as FF-A direct messages

This patch adds support for correctly receiving a request for a
Standalome MM driver service using the MM communication protocol
packaged as an FF-A direct request message.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: Ie59dc12e951e1de609010dd784606fde35d01c23