Platform/Sgi: Define values for ACPI table header

For ACPI tables that are generated dynamically, define the ACPI table
header values that have to be used to build the table header.

Co-authored-by: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: I54cbeac58028a5e4334cea24b36147d2b6606260