BeagleBoardPkg/PrePi: base GCD memory space size on CPU's PA range

BeagleBoardPkg/PrePi: base GCD memory space size on CPU's PA range

Derive the size of the GCD memory space map directly from the CPU's
information registers rather than from the PcdPrePiCpuMemorySize PCD,
which will be removed.

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>