MAINTAINERS: add exclude for dt-bindings to imx entry

Since the IMX (as in i.MX, the NXP SoCs) MAINTAINERS entry claims
everything that contains the name "imx", hanges to device tree
bindings for any Sony IMX image sensor are likely to be sent to the
maintainers listed therein. Add the missing exclude to fix that.

Fixes: da8b7f0f ("MAINTAINERS: add all files matching "imx" and "mxs" to the IMX entry")
Suggested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Michael Riesch <michael.riesch@collabora.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>