serial: uart_nrfx_uarte: graceful async power down

Gracefully shutdown the UARTE peripheral when the async API is used.
Failure to do so results in the driver being unusable when powered back
up as the required events (ENDTX & TXSTOPPED) are not set. This also
ensures that the last byte sent out via `poll_out` is properly output
on the serial line before powering down.

Fixes #31930.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>