drivers: serial: nrfx: Add pullups to RXD and CTS

Configured UART/UARTE input pins to have pullups. Otherwise when
uart is disconnected pins are floating and generate receiver
errors.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>