MdeModulePkg: Create PCDs to be used in support of SEV-ES

MdeModulePkg: Create PCDs to be used in support of SEV-ES

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

Two new dynamic MdeModulePkg PCDs are needed to support SEV-ES under OVMF:
  - PcdGhcbBase:       UINT64 value that is the base address of the GHCB
                       allocation.
  - PcdGhcbSize:       UINT64 value that is the size, in bytes, of the
                       GHCB allocation (size is dependent on the number of
                       APs).

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>