OvmfPkg: PlatformPei: reserve early page tables on X64

OvmfPkg: PlatformPei: reserve early page tables on X64

On X64, the reset vector code in
"OvmfPkg/ResetVector/Ia32/PageTables64.asm" identity maps the first 4GB of
RAM for PEI, consuming six frames starting at 8MB.

This range is declared by the PcdOvmfSecPageTablesBase/Size PCDs.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
[jordan.l.justen@intel.com: Move to MemDetect.c; use PCDs]
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15298 6f19259b-4bc3-4df7-8a09-765794883524