OvmfPkg/Sec: Move SEV-ES SEC workarea definition to common header file

OvmfPkg/Sec: Move SEV-ES SEC workarea definition to common header file

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3108

In order to allow for the SEV-ES workarea to be used for other purposes
and by other files, move the definition into the BaseMemEncryptSevLib
header file, MemEncryptSevLib.h.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <07d66f3384bd54da97d540e89b9f3473a6d17231.1610045305.git.thomas.lendacky@amd.com>