ARM: dts: at91: fixes uart pinctrl, set pullup on rx, clear pullup on tx

Remove pullup on uart TX signals, they are push-pull outputs thus
pullups are pointless.

Add pullup on uart RX signals, they prevent the RX signals to be left
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>