MAINTAINERS: add maintainer for i.MX DRM driver

Add myself as the maintainer of the i.MX DRM driver.

Signed-off-by: Philipp Zabel <[e-mail]>
Acked-by: Shawn Guo <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

MAINTAINERS

···
 F:	include/drm/exynos*
 F:	include/uapi/drm/exynos*
 
+DRM DRIVERS FOR FREESCALE IMX
+M:	Philipp Zabel <[e-mail]>
+L:	[e-mail]
+S:	Maintained
+F:	drivers/gpu/drm/imx/
+F:	Documentation/devicetree/bindings/drm/imx/
+
 DRM DRIVERS FOR NVIDIA TEGRA
 M:	Thierry Reding <[e-mail]>
 M:	Terje Bergström <[e-mail]>