drivers: serial: Add gpio management to low power mode

When low power mode is enabled then whenever UARTE is not active,
driver attempts to put the peripheral into the lowest power state
by stopping and disabling UARTE. However, it did not put pins into
sleep state which could lead to increased current consumption.
Adding pins state handling to the low power mode.

Pins are put into sleep state only if CONFIG_PM_DEVICE=y.

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