rockchip: use puts instead of printf when back to bootrom

printf will increase the code size more than 1kb, but platform
like rk3036 has no enough space for it.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>