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>