staging: mt7621-dts: use 'function' and 'group' for pinctrl bindings

According to documentation 'pinctrl-bindings.txt' bindings 'group'
and 'function' can be used directly. So replace all of them.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>