serial: atmel_usart: Fix early debug not work in SPL

Add the uart init function to be used on both probe and the early
debug uart init. For the latter, the input clock should be from
CONFIG_DEBUG_UART_CLOCK.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>