staging: mt7621-dts: use standard 'groups' instead of 'group' for pinctrl bindings

Pinctrl generic functions looks for standard property 'groups' in DT to get
data and use in the driver. Change all of them.

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