blk: Use a macro for the typical block size

Avoid using the magic number 512 directly.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/blk.h

@@ -21,6 +21,8 @@ typedef ulong lbaint_t;
 #define LBAF "%" LBAFlength "x"
 #define LBAFU "%" LBAFlength "u"
 
+#define DEFAULT_BLKSZ		512
+
 struct udevice;
 
 static inline bool blk_enabled(void)