Platform/Sgi: define memory region for GHES error status block

Allow platforms to define the base address and size of the memory region
that is reserved for MM drivers to populate the GHES generic error
status block with information about the platform error.

Co-authored-by: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: Id3343085a9c9cb9d5ad7db908e4ec41f747ddff8