Platform/Sgi: allow use of SDT protocol

Allow the use of ACPI system description table (SDT) protocol on Arm
reference design platforms. This protocol will be used for dynamic
addition of ACPI table such as the SDEI ACPI table.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: If39348d2289bced0b1e0d5142d11f3ab7b0f5b5e