drivers: dac: added driver for TI DACx3608

The DAC53608 and DAC43608 (DACx3608) are lowpower, eight-channel,
voltage-output, 10-bit or 8-bit digital-to-analog converters (DACs)
respectively. They support I2C with a wide power supply range
from 1.8 V to 5.5 V, and a full scale output voltage range of
1.8 V to 5.5 V. The DACx3608 also includes per channel, user
programmable, power down registers.

Signed-off-by: Matija Tudan <mtudan@mobilisis.hr>