MdePkg: Add PCCT table signature definition

MdePkg: Add PCCT table signature definition

The Platform Communications Channel Table (PCCT) was defined in:
ACPI Specification Version 5.0, Errata A - Published Nov. 13, 2013.

Starting from the Acpi50.h header file, there are definitions
describing the table but a macro with the table's signature is missing.

This patch adds the definition of Platform Communications Channel
Table's signature to the relevant ACPI header files.

Signed-off-by: Krzysztof Koch <krzysztof.koch@arm.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>