drivers/rtc/rtc-hym8563.c: add optional clock-output-names property

This enables the setting of a custom clock name for the clock provided by
the hym8563 rtc.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>