Finding the appropriate settings of meson plls is too tricky to be done
entirely at runtime, using calculation only. Many combination of m, n
and od won't lock which is why we are using a table for this.  However,
for plls having a fractional parameters, it is possible to improve on
the result provided by the table by calculating the frac parameter.

This change adds the calculation of frac when the parameter is available
and the rate provided by the table is not an exact match for the
requested rate.
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>