mips: dts: ralink: mt7621: remove defining gpio function for pin groups

All pin groups function as gpio unless set otherwise. Therefore, remove
this unnecessary binding.

Tested on UniElec U7621-06-16M on OpenWrt.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>