Add support to mkimage for rv1108 soc, the max
spl code size for rv1108 is 6kb, and the spl
code should be packed by rksd, wether boot from
emmc or spi nor flash.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>