serial: atmel-usart: add support for skiping debug UART init

serial: atmel-usart: add support for skiping debug UART init

Currently, atmel-usart does not respect CONFIG_DEBUG_UART_SKIP_INIT so
it will always configure the debug UART.

However, this is unwanted on platforms on which TF-A or some other firmware
has already configured the debug UART.

This will be used for Microchip LAN969x support, so simply return early if
CONFIG_DEBUG_UART_SKIP_INIT is set.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>