pinctrl: imx7ulp: constify struct imx_cfg_params_decode

The decode parameters are constant mark them const.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>