rtc: import ds1307 driver from linux-3.15 ...

Current ds1307 rtc driver supports only ds1307 and ds1338 chips;
it has no nvram support at the moment.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>