Include EL2 registers related to Nested Virtualization in EL2 context
save/restore routines if architecture supports it and platform wants to
use these features in Secure world.

Change-Id: If006ab83bbc2576488686f5ffdff88b91adced5c
Signed-off-by: Arunachalam Ganapathy <arunachalam.ganapathy@arm.com>