MAINTAINERS: add i.MX8 DT path to i.MX architecture

Shawn agreed to take patches for the i.MX8 parts through his tree.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>