rtc: move power of 2 periodic frequency check down into drivers

Move the power of 2 check on frequencies down into individual rtc drivers

This is to allow for non power of 2 real time clock periodic interrupts
such as those on the pxa27x to be found in the new pxa27x-rtc driver

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-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>

drivers/rtc/rtc-s3c.c

@@ -94,6 +94,9 @@ static int s3c_rtc_setfreq(struct device *dev, int freq)
 {
 	unsigned int tmp;
 
+	if (!is_power_of_2(freq))
+		return -EINVAL;
+
 	spin_lock_irq(&s3c_rtc_pie_lock);
 
 	tmp = readb(s3c_rtc_base + S3C2410_TICNT) & S3C2410_TICNT_ENABLE;