vexpress64: increase max gunzip size

vexpress64 kernels are usually over 8 MBytes in length, so setting the
max uImage length to 64 Mbytes should give us plenty of scope for
expansion.

I mostly chose this length to match other board configs that use
"(64 << 20)".

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

include/configs/vexpress_aemv8a.h

@@ -41,6 +41,8 @@
 #error "Unknown board variant"
 #endif
 
+#define CONFIG_SYS_BOOTM_LEN (64 << 20)      /* Increase max gunzip size */
+
 /* Flat Device Tree Definitions */
 #define CONFIG_OF_LIBFDT