MAINTAINERS: add "S: Maintained" to clkdev and clk sections

Russell King seems to be maintaining these.  Might as well mark them with
the appropriate S: entry

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>