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

Remove pullup on dbgu DTXD signal, it is a push-pull output thus the
pullup is pointless.

Add pullup on dbgu DRXD signal, it prevents the DRXD signal to be left
floating and so consuming a useless extra amount of power in crowbarred
state if nothing is externally connected to dbgu.

Signed-off-by: Sylvain Rochet <sylvain.rochet@finsecur.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>