MAINTAINERS: update email address and cleanup for exynos entry

Use kernel.org account instead of samsung.com and cleanup for Samsung
s3c, s5p and exynos SoCs.

Signed-off-by: Kukjin Kim <kgene@kernel.org>
Acked-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>