drivers: serial: nrfx_uarte: Refactor driver

Refactored driver to prepare for low power extension. Functional
change is limited to handling of RX_DISABLED event which is now
generated from RXTO interrupt context after RX is stopped. Previously,
RX was not stopped after the transfer.

Rx flushing function contains hardware limitation workaround.
Workaround is applied only if flushed data is not discarded.

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