drivers: serial: nrfx_uarte: Support for low power polling mode

Add support for getting to the lowest power mode when polling is
used with disable-rx property and interrupts are not used for
that UARTE. So far disabling of the UARTE peripheral was done in
the interrupt but in some cases interrupt may not be available
and in that case uart_poll_out shall wait until byte is transferred
and put UARTE into the lowest power state.

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