UefiCpuPkg: Create an SEV-ES workarea PCD

UefiCpuPkg: Create an SEV-ES workarea PCD

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

Create an SEV-ES workarea PCD. This PCD will be used for BSP communication
during SEC and for AP startup during PEI and DXE phases, the latter is the
reason for creating it in the UefiCpuPkg.

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>