ARM: dts: at91rm9200: pullup rx on uart0

For consistency with all other serial pins, add this pullup. It also
prevents the signal from floating and so consuming a useless extra amount
of power in crowbarred state if nothing is connected to RX.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>