debug_uart: Support board-specific UART initialisation

debug_uart: Support board-specific UART initialisation

Some boards need to set things up before the debug UART can be used. On
these boards a call to debug_uart_init() is insufficient. When this option
is enabled, the function board_debug_uart_init() will be called when
debug_uart_init() is called. You can put any code here that is needed to
set up the UART ready for use, such as set pin multiplexing or enable
clocks.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>