rockchip: rk3308: Drop unused rk_board_init()

Nothing is calling the function rk_board_init() and the io-domain driver
can handle the functions intended purpose based on information from DT.

Cleanup by removing the unused rk_board_init() function and re-sort
included headers.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>