The DS1307 has a square wave output pin, which can be used to output a
clock signal from the DS1307.  Additionally, the DS1308 supports
configuring this pin as an input from an external clock source to
which it should sync itself.

Add support with OF device tree properties to configure these
settings.  Supported features are using the clock pin as an output, an
input, the rate of the pin, and if it should be enabled on battery
backup power.

The driver does not check that the selected features are supported by
the clock chip being used.  It is the designer's responsibility to
create a valid device tree node; the bootloader does not attempt to be
a device tree validator.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>