ArmPkg/ArmMmuLib: base page table VA size on GCD memory map size

ArmPkg/ArmMmuLib: base page table VA size on GCD memory map size

As reported by Eugene, the practice of sizing the address space in the
virtual memory system based on the maximum address in the table passed
to ArmConfigureMmu() is problematic, since it fails to take into account
the fact that the GCD memory space may be extended at a later time, both
for memory and for MMIO. So instead, choose the VA size identical to the
GCD memory map size, which is based on PcdPrePiCpuMemorySize on ARM
systems.

Reported-by: Eugene Cohen <eugene@hp.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Eugene Cohen <eugene@hp.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>