rtc: add driver for BQ32000 I2C RTC

This patch adds basic support for Texas Instruments BQ32000 I2C RTC.  Only
time reading/writing is implemented.  Advanced features, such as trickle
charger and crystal calibration are not supported.

Signed-off-by: Piotr Ziecik <kosmo@semihalf.com>
Signed-off-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>