[amlogic][cpu-freq] Add CPU Freq. Scaling support

- Added support for configuring the dynamic mux
to generate frequencies less than 1GHz using MPLL.
- Cleanup and bug fixes in HIU PLL library.

Test: Used clkctl utility to verfiy freq. changes are
actually applied.

Change-Id: I9ef0da66c73b15814b566e5ab07a565ac08b875a