Allow the use of OFW I2C bus together with iicbb(4) on OFW-based systems.

This change makes ofw_iicbus attach to iicbb(4) controllers in addition to
the already supported i2c host bridges (iichb).

On iicbb(4) allow the direct access of the OFW parent node by its children,
so they can be directly attached to iicbb(4) node on the DTS without the
need of describing the i2c bus.

Approved by:	adrian (mentor, implicit)