spl: imx6: Provide a SPL_SIZE_LIMIT default

As explained in include/configs/imx6_spl.h. the i.MX6 SPL
size limit is 68KB (4KB header + 64KB max size).

Provide such limit for all i.MX6 boards that use SPL
to detect SPL size overflow in build time.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index ac2f470..48f6b39 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -28,6 +28,7 @@
 config SPL_SIZE_LIMIT
 	int "Maximum size of SPL image"
 	depends on SPL
+	default 69632 if ARCH_MX6
 	default 0
 	help
 	  Specifies the maximum length of the U-Boot SPL image.