rtc: ds1307: add support for watchdog timer on ds1388

The DS1388 variant has watchdog timer capabilities. When using a DS1388
and having enabled CONFIG_WATCHDOG_CORE register a watchdog device for
the DS1388.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200330025500.6991-1-chris.packham@alliedtelesis.co.nz
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>