ArmPkg/MmCommunicationDxe: Discover the StMM SP

This patch adds support for discovering the presence of the SP using the
EFI_MM_COMMUNICATION_PROTOCOL GUID that implements Standalone MM
drivers. This is done by querying the framework through
FFA_PARTITION_INFO_GET whether any partition that implements the
EFI_MM_COMMUNICATION_PROTOCOL is present or not. The partition ID and
its properties are stashed for use in subsequent communication with the
StMM SP.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: If10884cb25c0b3ca8c8a76a0b1348f92b819c425