viafb: allow some pll calculations

This patch allows calculating the pll multiplier within limits based
on the previous table. All available information supports that it
should be possible/sane to choose the multiplier free within some
ranges.
Storing the multiplier ranges instead of lots of pll configurations
reduces the memory needed and may as well improve the performance.
It is also expected to provide better pll values resulting in better
frequencies for the connected devices.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>