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: I2704c91f61c1fb02987bdcecd7c6a5b9553d9a96