drivers: serial: nrfx_uarte: Add low power mode

Lowest power consumption can be achieved when uarte peripheral
is disabled when not used. In low power mode, need for both
directions is tracked and if both are no in use peripheral is
disabled. TX disabling is instant but RX requires flushing RX
fifo because data in hardware fifo is lost when peripheral is
re-enabled.

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