staging: mt7621-pinctrl: make use of pinctrl_utils_free_map

There was a custom 'rt2880_pinctrl_dt_free_map' function which
it was doing the same as pinctrl_utils_free_map defined in
'pinctrl-utils.h' header file. Use it instead avoiding
code duplications.

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