drivers: serial: nrf_uarte: Fix power management transitions

Deadlock may occur when uart device was put to low power state
while uart_poll_out was in progress. Poll out pends until uarte
is ready to send new byte and it is detected by endtx and txstopped
events being set. When uarte was disabled while poll_out was pending
events were never set and that lead to deadlock. Added a step
in going to low power state in which CPU waits for txstopped event
and only after that uarte peripheral is disabled.

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