UefiCpuPkg: Create PCD to be used in support of SEV-ES

UefiCpuPkg: Create PCD to be used in support of SEV-ES

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

A new dynamic UefiCpuPkg PCD is needed to support SEV-ES under OVMF:
  - PcdSevEsIsEnabled: BOOLEAN value used to indicate if SEV-ES is enabled

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>