uboot-imx

rockchip: rk3188: use board_debug_uart_init() for UART io init

Sync with other rockchip SoCs, use board_debug_uart_init() to
init default UART iomux.

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