drm/panthor: Add an entry to MAINTAINERS

Add an entry for the Panthor driver to the MAINTAINERS file.

v4:
- Add Steve's R-b

v3:
- Add bindings document as an 'F:' line.
- Add Steven and Liviu as co-maintainers.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>