drivers: serial: nrfx_uarte: Fix pin retention

b6d45423 Added support for pin retention but it was added
only to the case when device PM is used. There is another mode
in which UARTE is disabled when idle (low power mode) and in
that case pin retention must also be added.

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