rockchip: rk3308: no DEBUG_UART_BOARD_INIT for ROCK Pi S

rockchip: rk3308: no DEBUG_UART_BOARD_INIT for ROCK Pi S

Call to board_debug_uart_init() is useless, as mainline U-Boot can
not build TPL for rk3308, and proprietary ddr.bin to be used as TPL
is responsible to init debug uart. Moreover current implementation
of board_debug_uart_init() is not compatible with ROCK Pi S, as it
sets pins for UART2 channel 1 breaking access to sdmmc due to pinmux
conflict. Debug uart for ROCK Pi S is UART0.

Thus, avoid ROCKCHIP_RK3308 to select DEBUG_UART_BOARD_INIT and allow
to deselct it in rock-pi-s-rk3308_defconfig. The DEBUG_UART_BOARD_INIT
is already implied by ARCH_ROCKCHIP, therefore other boards based on
rk3308 chip are not affected by change.

Signed-off-by: Massimo Pegorer <massimo.pegorer@vimar.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>