drivers: serial: Add uart_async_rx module

Add module which can handle RX path of UART asynchronous RX API. Module
can be utilized in cases where processing of received data is not performed
directly in the event context but it is delayed. At least two use cases
has been identified (shell async UART backend, asynchronous to interrupt
driven adaptation layer).

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