drivers: serial: nrfx_uarte: Fix use of PM_DEVICE_ISR_SAFE

PM_DEVICE_ISR_SAFE shall not be used when non-asynchronous API is used
because RX is disabled in suspend action and that takes relatively
long time. In case of PM_DEVICE_ISR_SAFE it is done with interrupts
disabled. RX is not used at all if disable-rx property is set and in
that case PM_DEVICE_ISR_SAFE can be used.

Added macro which determines if ISR safe mode can be used.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
(cherry picked from commit e0f5241b)