ArmPkg: Introduce support for PcdFfaEnable

ArmPkg: Introduce support for PcdFfaEnable

The Secure Partition(SP) can request services from the Secure
Partition Manager Core(SPMC) either through FF-A calls or through the
existing SVC calls. Add a feature flag Pcd for enabling the FF-A
method -- when this is set to FALSE, the SP uses the existing SVC
calls for making the requests.

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