ArmPlatformPkg: retreive error source descriptors from MM

Add a driver that retreives error source descriptors from MM and
populates those into the HEST ACPI table. The error source descriptors
that are available from the MM side are retreived using MM Communicate 2
protocol.

The first call into the MM returns the size of MM Communicate buffer
required to hold all error source descriptor info. The communication
buffer of that size is then allocated and the second call into MM
returns the error source descriptors in the communication buffer.
The retreived error source descriptors are then appended to the HEST
table.

Co-authored-by: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: I6355616db838c1a0720dc783c92c9bd907ed4d18