rtc-at91rm9200: remove now-unneeded code

The non-functional periodic IRQ support was previously removed from the
AT91RM9200 RTC driver.  Remove the remaining AT91_RTC_FREQ definition.

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Cc: David Brownell: <david-b@pacbell.net>
Cc: Alessandro Zummo: <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>