Move ALLOC_CACHE_ALIGN_BUFFER() to the new memalign.h header

Now that we have a new header file for cache-aligned allocation, we should
move the stack-based allocation macro there also.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/rpmb.c b/drivers/mmc/rpmb.c
index 9d0b8bc..1c6888f 100644
--- a/drivers/mmc/rpmb.c
+++ b/drivers/mmc/rpmb.c
@@ -10,6 +10,7 @@
 
 #include <config.h>
 #include <common.h>
+#include <memalign.h>
 #include <mmc.h>
 #include <u-boot/sha256.h>
 #include "mmc_private.h"