[PATCH] rtc: small documentation update

Rtc driver documentation update

* Mention the max-user-freq control file.
* Add missing header in example code.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>