Platform/Sgi: dmc-620 firmware-first error handling

Enable the use of HEST table generation protocol, GHES error source
descriptor protocol and DMC-620 MM driver on ARM Neoverse Reference
Design platforms. This allows firmware-first error handling and
reporting of DMC-620 memory controller's 1-bit DRAM ECC errors.

Co-authored-by: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: Ie6fd56aca13042aaa65a959c53eedb55913d9cdd