staging: mt7621-pinctrl: make use of pinctrl_utils_reserve_map

Function rt2880_pinctrl_dt_node_to_map was using 'kzalloc' to
reserve map memory. There is a 'pinctrl_utils_reserve_map' to
do this function. Just use it.

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