refactor(el3-spmc): check emad_count offset

Subsequent code will assume that it version-independent, so check it
with a CASSERT.

Change-Id: I233b51ef700103f1a0789d5608e3b02c96d0eeb7
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>

include/services/el3_spmc_ffa_memory.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2022-2023, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -254,5 +254,8 @@ struct ffa_mtd {
 	uint64_t reserved_40_47;
 };
 CASSERT(sizeof(struct ffa_mtd) == 48, assert_ffa_mtd_size_mismatch);
+CASSERT(offsetof(struct ffa_mtd, emad_count) ==
+	offsetof(struct ffa_mtd_v1_0, emad_count),
+	assert_ffa_mtd_emad_count_offset_mismatch);
 
 #endif /* EL3_SPMC_FFA_MEM_H */