OvmfPkg: Move SEC/PEI Temporary RAM from 0x70000 to 0x810000

OvmfPkg: Move SEC/PEI Temporary RAM from 0x70000 to 0x810000

Note: The Temporary RAM memory size is being reduced from
      64KB to 32KB. This still appears to be more than
      adequate for OVMF's early PEI phase. We will be adding
      another 32KB range of RAM just above this range for
      use on S3 resume.

The range is declared as part of MEMFD, so it is easier
to identify the memory range.

We also now assign PCDs to the memory range.

The PCDs are used to set the initial SEC/PEI stack in
SEC's assembly code.

The PCDs are also used in the SEC C code to setup
the Temporary RAM PPI.

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@15147 6f19259b-4bc3-4df7-8a09-765794883524