ArmPkg/ArmPkg.dec: Redefined PcdSystemMemory(Base|Size) as UINT64

ArmPkg/ArmPkg.dec: Redefined PcdSystemMemory(Base|Size) as UINT64

The System Memory region might be out of the 32-bit memory space.

This change has been validated on the FVP AArch64 model using 4GB
of DRAM at 0x8_0000_0000:

-  # System Memory (2GB)
-  gArmTokenSpaceGuid.PcdSystemMemoryBase|0x80000000
-  gArmTokenSpaceGuid.PcdSystemMemorySize|0x80000000
+  # System Memory (4GB)
+  gArmTokenSpaceGuid.PcdSystemMemoryBase|0x800000000
+  gArmTokenSpaceGuid.PcdSystemMemorySize|0x100000000

EFI Shell and Linux kernel boot successfully.

Note: This change has not been validated on AArch32. I expect some
early assembly code to not work.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15093 6f19259b-4bc3-4df7-8a09-765794883524