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>

OvmfPkg/Include/Library/MemEncryptSevLib.h

@@ -2,7 +2,7 @@
 
   Define Secure Encrypted Virtualization (SEV) base library helper function
 
-  Copyright (c) 2017, AMD Incorporated. All rights reserved.<BR>
+  Copyright (c) 2017 - 2020, AMD Incorporated. All rights reserved.<BR>
 
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
@@ -13,6 +13,20 @@
 
 #include <Base.h>
 
+//
+// Internal structure for holding SEV-ES information needed during SEC phase
+// and valid only during SEC phase and early PEI during platform
+// initialization.
+//
+// This structure is also used by assembler files:
+//   OvmfPkg/ResetVector/ResetVector.nasmb
+//   OvmfPkg/ResetVector/Ia32/PageTables64.asm
+// any changes must stay in sync with its usage.
+//
+typedef struct _SEC_SEV_ES_WORK_AREA {
+  UINT8    SevEsEnabled;
+} SEC_SEV_ES_WORK_AREA;
+
 /**
   Returns a boolean to indicate whether SEV-ES is enabled.