rtc: add max_user_freq to sysfs

drivers/char/rtc.c exposed a sysctl to change the maximum frequency at
which a non-root user could ask the RTC to generate interrupts (via the
RTC_IRQP_SET ioctl).  This value is no longer available under the new RTC
subsystem, so add it to sysfs for each RTC device.

Works for me on x86_64 (both reads and writes), using rtc-cmos.

Signed-off-by: Bryan Kadzban <bryan@kdzbn.homelinux.net>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>