ArmPkg: Add PCDs needed for MM communication driver.
ArmPkg: Add PCDs needed for MM communication driver.

This patch defines PCDs to describe the base address and size of
communication buffer between normal world (uefi) and standalone MM
environment in the secure world.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Sughosh Ganu <89c5e32bab4ac30865936b46fb918f58cd11fd98@arm.com>
Reviewed-by: Ard Biesheuvel <66d3c5fdaeea7ff1f996ad04f2c45e08ab38e2f5@linaro.org>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2