pinctrl: mvebu: armada-xp: fix binding documentation of ge1 pins

There was an off-by-one in the documentation of the ge1(txd[0-3])
pins, which is fixed by this commit. Since the driver was correct, and
the subnames are anyway not used in the DT binding itself, there is no
need to push this documentation fix for stable.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>