pinctrl: fsl: imx7ulp: change to use imx legacy binding

We already had an earlier conclusion that all new i.MX Socs will keep
using the legacy i.MX Pinctrl bindings instead of generic pin config.
However, MX7ULP generic pin config binding support has already been in
tree before that time. Per SoC maintainers' suggestions, in order to
get a better consistency for all i.MX devices, we'd like to go back to
imx legacy binding for MX7ULP as well.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>