Add a new driver, ds13rtc, that handles all DS13xx series i2c RTC chips.

Add a new driver, ds13rtc, that handles all DS13xx series i2c RTC chips.

This driver supports only basic timekeeping functionality.  It completely
replaces the ds133x driver.  It can also replace the ds1374 driver, but that
will take a few other changes in MIPS code and config, and will be committed
separately.  It does NOT replace the existing ds1307 driver, which provides
access to some of the extended features on the 1307 chip, such as controlling
the square wave output signal.  If both ds1307 and ds13rtc drivers are
present, the ds1307 driver will outbid and win control of the device.

This driver can be configured with FDT data, or by using hints on non-FDT
systems.  In addition to the standard hints for i2c devices, it requires
a "chiptype" string of the form "dallas,ds13xx" where 'xx' is the chip id
(i.e., the same format as FDT compat strings).