Added spread spectrum parameter calculation and setting

[paolo-sz]

an empiric fix on PLL reset (it was not working without some magic undocumented value) is also included.

These changes are going to cover the case where the external SSEN pin is set to high (spread spectrum enabled). If the related registers are not set correctly the PLL A does not lock. The reset fix is somehow related to the spread spectrum changes because otherwise I was not able to recover from the initial PLL A not locked state.

See the comments within code.

[NT7S]

Thanks much, I'll try to evaluate it with my hardware soon.