cpu/x86/smm: Add define for AMD64 save area

Create an SMM_AMD64_SAVE_STATE_OFFSET #define similar to others in the
same file.

Change-Id: I0a051066b142cccae3d2c7df33be11994bafaae0
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/21499
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/include/cpu/x86/smm.h

@@ -37,7 +37,8 @@
  * starts @ 0x7e00
  */
 #define SMM_AMD64_ARCH_OFFSET 0x7e00
-
+#define SMM_AMD64_SAVE_STATE_OFFSET \
+	SMM_SAVE_STATE_BEGIN(SMM_AMD64_ARCH_OFFSET)
 typedef struct {
 	u16	es_selector;
 	u16	es_attributes;