rtc: add m41t62 support to rtc-m41t80 driver

Compared to the other supported chips, the m41t62 uses a different
register to set the square wave frequency.

Signed-off-by: Daniel Glockner <dg@emlix.com>
Cc: Chris Zankel <chris@zankel.net>
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>