rtc-ds1307 exports NVRAM

Export the NVRAM on DS1307 and DS1338 chips, like several of the
other drivers do for such combination RTC-and-NVRAM chips.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>