ArmPkg/MmCommunicationDxe: Add support for obtaining FF-A partition ID

This patch invokes the FFA_ID_GET to obtain and stash the ID of the the
FF-A partition that implements the DXE MM communication driver. This ID
is used in subsequent patches for sending and receiving MM communication
protocol requests and responses that are packaged as FF-A messages.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I5f20fe11f206a0c0ddaf87a2b5bba0bd5448d5fd