watchdog: st_wdt: 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 Watchdog driver
to encompass Clocksource.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>