rockchip: Move bootrom-related declarations to a header

So far spl-boards have declared the back_to_brom() function as simple
extern in the files themself. That doesn't scale well if every boards
defines this on its own.
Therefore move the declarations to a bootrom header.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Kever Yang <kever.yang@rock-chips.com>