drivers: misc: coresight: nrf_etr: Imply UART asynchronous API

Asynchronous API is recommended to be used by ETR buffer handler but
polling is also supported. Imply UART_ASYNC_API so that it is possible
to disable it in project config and fall back to polling if needed. For
example to reduce memory footprint.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>