ArmPkg/ArmLib: add support for reading the max physical address space size

Add a helper function that returns the maximum physical address space
size as supported by the current CPU.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

ArmPkg/Include/Library/ArmLib.h

@@ -733,4 +733,10 @@ ArmWriteCntvOff (
   UINT64   Val
   );
 
+UINTN
+EFIAPI
+ArmGetPhysicalAddressBits (
+  VOID
+  );
+
 #endif // __ARM_LIB__