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>

MAINTAINERS

@@ -1689,6 +1689,7 @@ M:	Krzysztof Kozlowski <krzk@kernel.org>
 R:	Javier Martinez Canillas <javier@osg.samsung.com>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 L:	linux-samsung-soc@vger.kernel.org (moderated for non-subscribers)
+Q:	https://patchwork.kernel.org/project/linux-samsung-soc/list/
 S:	Maintained
 F:	arch/arm/boot/dts/s3c*
 F:	arch/arm/boot/dts/s5p*