StandaloneMmPkg: Add global check for FF-A abis

This patch copies the value of the corresponding stack variable to a global
variable so that it can be used to determine whether FF-A v1.1 or earlier ABIs
should be used for communication with the SPMC.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: I1eca53f58ca8982f5e729a2fd385323528628307