MAINTAINERS: Add exynos filename match to ARM/S5P EXYNOS ARM ARCHITECTURES

This will help direct patches related to exynos to the
linux-samsung-soc mailing list for discussion.

Suggested by Joe Perches.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>