rockchip: rk3368: move board_debug_uart_init() to rk3368.c

rockchip: rk3368: move board_debug_uart_init() to rk3368.c

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 later for all rockchip SoCs.

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