rtc: ds1307: Ensure oscillator is enabled for DS1388

Similar to the other variants the DS1388 has a bit to stop the
oscillator to reduce the power consumption from VBAT. Ensure that the
oscillator is enabled when the system is up.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200816235731.21071-1-chris.packham@alliedtelesis.co.nz