MAINTAINERS: add Freescale pin controllers

Add Dong Aisheng, Fabio Estevam, Shawn Guo and myself as maintainer
and the Pengutronix kernel team as reviewer.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>