StandaloneMmPkg: Add option to use FF-A calls for getting SPM version

With the introduction of Firmware Framework(FF-A), a Secure Partition
can get the SPM version either using FF-A calls or through the
existing svc calls. Use a runtime check to use either of the two
methods based on the Pcd feature flag value.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Co-developed-by: Achin Gupta <achin.gupta@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>

StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/StandaloneMmCoreEntryPoint.inf

@@ -51,5 +51,8 @@
   gEfiStandaloneMmNonSecureBufferGuid
   gEfiArmTfCpuDriverEpDescriptorGuid
 
+[FeaturePcd.AARCH64]
+  gArmTokenSpaceGuid.PcdFfaEnable
+
 [BuildOptions]
   GCC:*_*_*_CC_FLAGS = -fpie