ARM: dts: armada-370: Update the mpp63 function in the device tree on Armada 370

Since the commit a526973e ("pinctrl: mvebu: Fix mapping of pin
63 (gpo -> gpio)"), the mpp63 is no more declared as a GPO but is a
GPIO. Even if in the datasheet this pin is described as GPO, the
experience of the D-Link DNS-327L board shows that it can be used as a
GPIO.

This commits generated warnings for the board using this pin as gpo, with
this patch the dts are fixed by using the new function (gpio) instead of
the old one.

The binding documentation has also been updated accordingly.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>