ucs: refactor clock sources/frequencies
ucs: refactor clock sources/frequencies

Start to decouple the source enumeration from ACLK, since the same sources
can be used for most clocks, and the frequencies are the same regardless of
which clock uses them.  This is a step toward issue #30.

pabigot/bsp430,pabigot/bsp430,pabigot/bsp430,pabigot/bsp430