MIPS: tl-wdr4300: remove debug_uart_init call

MIPS: tl-wdr4300: remove debug_uart_init call

In order to add a generic MIPS debug_uart_init call right before the call to
board_early_init_f, we need to remove all calls to debug_uart_init from every
MIPS boards.
WDR4300 doesn't provide a board_debug_uart_init and configures pinmux in
board_early_init_f instead. Since I have no idead of what's the needed uart
pinmux config, I copied the whole pinmux config to a new function that is
called from board_early_init_f if CONFIG_DEBUG_UART_BOARD_INIT is not enabled.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>