OvmfPkg: PlatformPei: enable larger permanent PEI RAM

OvmfPkg: PlatformPei: enable larger permanent PEI RAM

We'll soon increase the maximum guest-physical RAM size supported by OVMF.
For more RAM, the DXE IPL is going to build more page tables, and for that
it's going to need a bigger chunk from the permanent PEI RAM.

Otherwise CreateIdentityMappingPageTables() would fail with:

> DXE IPL Entry
> Loading PEIM at 0x000BFF61000 EntryPoint=0x000BFF61260 DxeCore.efi
> Loading DXE CORE at 0x000BFF61000 EntryPoint=0x000BFF61260
> AllocatePages failed: No 0x40201 Pages is available.
> There is only left 0x3F1F pages memory resource to be allocated.
> ASSERT .../MdeModulePkg/Core/DxeIplPeim/X64/VirtualMemory.c(123):
> BigPageAddress != 0

(The above example belongs to the artificially high, maximal address width
of 52, clamped by the DXE core to 48. The address width of 48 bits
corresponds to 256 TB or RAM, and requires a bit more than 1GB for paging
structures.)

Cc: Maoming <maoming.maoming@huawei.com>
Cc: Huangpeng (Peter) <peter.huangpeng@huawei.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Brian J. Johnson <bjohnson@sgi.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Brian J. Johnson <bjohnson@sgi.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17719 6f19259b-4bc3-4df7-8a09-765794883524