ArmPlatformPkg/MemoryInitPeiLib: revert "don't reserve primary FV in memory"

Commit 8ae5fc182941 ("ArmPlatformPkg/MemoryInitPeiLib: don't reserve
primary FV in memory") deleted the code that removes the memory covering
the primary firmware volume from the memory map. The assumption was that
this is no longer necessary now that we no longer expose compression and
PE/COFF loader library code from the PrePi module to DXE core.

However, the FV is still declared, and so code may attempt to access it
anyway, which may cause unexpected results depending on whether the
memory has been reused for other purposes in the mean time. So revert
this change in preparation of dealing with this in a better way.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>