pinctrl: meson: add kerneldoc, use pinconf_generic_dt_node_to_map_pin()

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
pinctrl: meson: rewrite DT bindings documentation

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>