ArmPlatformPkg/MemoryInitPei: Check if the main System Memory resource has been declared

ArmPlatformPkg/MemoryInitPei: Check if the main System Memory resource has been declared

The PlatformLib could have declared the resource hob for
the main system memory region.
This change would prevent to get this system memory resource
to be declared twice.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <Olivier.Martin@arm.com>
Reviewed-by: Ronald Cron <Ronald.Cron@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17834 6f19259b-4bc3-4df7-8a09-765794883524