da9063_iic: Add new driver for the Dialog Semiconductor DA9063 PMIC

This is an MFD with regulators, an RTC and a watchdog, among other
things. This adds the necessary infrastructure for specific children to
be added.

Note that the PMIC can also be attached via SPI, not just I2C, and so
the interface is abstracted. No SPI implementation is added, however.

This is the PMIC on SiFive's HiFive Unmatched.

Reviewed by: manu, mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D36199