MAINTAINERS: Add wpan patchwork

This patchwork instance is hosted on kernel.org and has been used for a
long time already, it was just not mentioned in MAINTAINERS.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20230411090122.419761-2-miquel.raynal@bootlin.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>