clk_fixed: Fix support for fixed factor clocks

[jmcneill]

Summary

This patch changes the following in clk_fixed:

• Use a different device description for fixed and fixed factor clocks
• Fix a bug where the "clock-div" property for fixed factor clocks was stored in the "mult" field of the clock definition.
• Get the fixed factor parent clock by index as there may not be a "clock-names" property on a fixed clock.