zynq: Extend kernel image size to 20MB

Extend max kernel image size. Gunzip is checking
this value. If kernel is larger, message below is shown.

Uncompressing Kernel Image ... Error: inflate() returned -5
GUNZIP: uncompress, out-of-mem or overwrite error -
must RESET board to recover

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

include/configs/zynq-common.h

@@ -221,6 +221,9 @@
 #define CONFIG_FIT_SIGNATURE
 #define CONFIG_RSA
 
+/* Extend size of kernel image for uncompression */
+#define CONFIG_SYS_BOOTM_LEN	(20 * 1024 * 1024)
+
 /* Boot FreeBSD/vxWorks from an ELF image */
 #if defined(CONFIG_ZYNQ_BOOT_FREEBSD)
 # define CONFIG_API