MAINTAINERS: Add Patchwork URL to Samsung Exynos entry

I use Patchwork for handling incoming patches. Put its address here so
submitters could know what is in the queue.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Kukjin Kim <kgene@kernel.org>