Improve calculation of MMCM multiplier and divisor values

[rohitk-singh]

The current code calculation of multiplier and divisor values for 7-Series (i.e, with MMCM) is not robust, and can fail (for example when setting resolution to 800x600@75.00Hz).

We need to improve the code so the calculated multiplier and divisor values are such that VCO frequency falls in the allowed range of 600MHz-1200MHz.

[mithro]

Rohit - see timvideos/litex-buildenv#157