[U-Boot] rockchip: add support for backing to bootrom download mode

[Philipp Tomsich]

> Rockchip bootrom will enter download mode if it returns from
> spl/tpl with a none-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/tps code override the interrupt settings.
> 
> Signed-off-by: Andy Yan <andy.yan at rock-chips.com>
> Reviewed-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
>  arch/arm/include/asm/arch-rockchip/bootrom.h |  2 +-
>  arch/arm/mach-rockchip/Kconfig               | 13 +++++++
>  arch/arm/mach-rockchip/bootrom.c             |  2 +-
>  arch/arm/mach-rockchip/save_boot_param.S     | 57 +++++++++++++++++++++++-----
>  4 files changed, 63 insertions(+), 11 deletions(-)
> 

Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>