mmc: Define a constant for the maximum block size

The number 512 appears quite a bit in the mmc code. Add a constant for this
so that it can be used here and in other parts of the code (e.g. SPL code
which loads from mmc).

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@google.com>

include/mmc.h

@@ -207,6 +207,9 @@
 #define PART_ACCESS_MASK	(0x7)
 #define PART_SUPPORT		(0x1)
 
+/* Maximum block size for MMC */
+#define MMC_MAX_BLOCK_LEN	512
+
 struct mmc_cid {
 	unsigned long psn;
 	unsigned short oid;