rockchip: make boot_mode related codes reused across all platforms

setup_boot_mode function use the same logic but different
mode register address across all the rockchip platforms,
so it's better to make this function reused across all the
platforms, and let the mode register address setting from
the config file.

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