rtc: st: Update IP layout information to include Clocksource

Initial submission adding support for this IP only included Watchdog and
the Real-Time Clock.  Now the third (and final) device is enabled this
trivial patch is required to update the comment in the RTC driver to
encompass Clocksource.

Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>