ArmPkg/ArmMmuLib: Revert "use a pool allocation for the root table"

ArmPkg/ArmMmuLib: Revert "use a pool allocation for the root table"

This reverts commit d32702d2c2aa23e828363a7f88829b78ce36c3af.

Using a pool allocation for the root translation table seemed like
a good idea at the time, but as it turns out, such allocations are
handled in a way that makes them unsuitable for this purpose: they
are backed by HOBs that don't remain in the same place during the
various PI phase changes, which means the address programmed into
the TTBR register is no longer valid, and may refer to memory that
is reported as available to the OS.

So switch back to using a page based allocation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>