rtc: add MAX8907 RTC driver

The MAX8907 is an I2C-based power-management IC containing voltage
regulators, a reset controller, a real-time clock, and a touch-screen
controller.

The driver is based on an original by or fixed by:
* Tom Cherry
* Prashant Gaikwad
* Joseph Yoon

During upstreaming, I (swarren):
* Converted to regmap.
* Fixed handling of RTC_HOUR register containing 12.
* Fixed handling of RTC_WEEKDAY register.
* General cleanup.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Cc: Tom Cherry <tcherry@nvidia.com>
Cc: Prashant Gaikwad <pgaikwad@nvidia.com>
Cc: Joseph Yoon <tyoon@nvidia.com>
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>