drivers: serial: nrfx_uarte: Add support for non ISR PM mode

When fast UARTE instance is used (e.g. UARTE120 in nrf54h20), PM actions
are not ISR safe because they include communication over IPC so they can
only be called from the thread context. Extend driver to support both
PM modes. When non ISR mode is used then uart_rx_enable() and uart_tx()
will return error if they are called from ISR and resume operation
would need to be called because device is suspended. On completion,
driver is calling pm_device_runtime_put_async which can be called from
the ISR context.

Additionally, suspending in the TXSTOPPED and RXTO events has been
moved after user callback. It allows to support the case where
uart_rx_enable() or uart_tx() are called from that callback context.
Since suspending is called after returning from the callback it will
not trigger suspend action because API called in the callback context
will increment the usage counter (when pm_device_runtime_get() is
called).

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