rockchip: check download key before bootup

Enter download mode if the download key pressed.

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>
[Converted printfs in boot_mode.c to debug/pr_err:]
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>