Platform/Sgi: enable support for UEFI secure boot

Enable the use of UEFI secure boot for Arm's Neoverse reference design
platforms. The UEFI authenticated variable store uses NOR flash 2 which
is accessible from Standalone MM context residing in a secure partition.

Signed-off-by: Sayanta Pattanayak <sayanta.pattanayak@arm.com>
Reviewed-by: Thomas Abraham <thomas.abraham@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>