Fix support for fixed factor clocks.

• Use a different device description for fixed and fixed factor clocks.
• Fix a bug where the "clock-div" property was stored in the "mult" field of the clock definition.
• Get the fixed factor parent clock by index instead of by name, as a clock-names property is not required to be present here.

Reviewed by: mmel, adrian (mentor)
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5703