ArmPkg/ArmMmuLib: take the CPU supported maximum PA space into account

ArmPkg/ArmMmuLib: take the CPU supported maximum PA space into account

In preparation of dropping PcdPrePiCpuMemorySize entirely, base the
maximum size of the identity map on the capabilities of the CPU.
Since that may exceed what is architecturally permitted when using
4 KB pages, take MAX_ADDRESS into account as well.

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