microblaze: Setup reasonable maximum bootm len

We are far from 8MB default size. Setup 64MB for now.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

include/configs/microblaze-generic.h

@@ -13,6 +13,8 @@
 /* MicroBlaze CPU */
 #define	MICROBLAZE_V5		1
 
+#define CONFIG_SYS_BOOTM_LEN	(64 * 1024 * 1024)
+
 /* linear and spi flash memory */
 #ifdef XILINX_FLASH_START
 #define	FLASH