rockchip: add support for enter to bootrom download mode

Rockchip bootrom will enter download mode if it returns from
spl/tpl with a non-zero value and couldn't find a valid image
in the backup partition.
This patch provide a method to instruct the system to back to
bootrom download mode by checking the BROM_DOWNLOAD_FLAG register.
As the bootrom download function relys on some modules such as
interrupts, so we need to back to bootrom as early as possbile
before the tpl/spl code override the interrupt configurations.

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