mfd: rtc-twl4030 driver

This adds a driver for the RTC inside the TWL4030 multi-function device.
It's a fairly basic RTC, with a wake-capable alarm.

Note that many of the pre-release Overo boards now in circulation can't
effectively use this RTC, because of a wiring error that puts its TWL
chip into "secure" mode.  (As in "secure yourself against tampering".)
This isn't an issue on other OMAP3 boards now supported in mainline,
such as Beagle and Labrador.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>