rockchip: rk3188: add board_debug_uart_init()

rockchip: rk3188: add board_debug_uart_init()

Use board_debug_uart_init() for UART iomux init instead of
do it in board_init_f, and move the function to soc file so
that we can find all the soc/board setting in soc file and
use a common board file.

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