clk: meson: gxbb: add the fractional part of the fixed_pll

The fixed_pll of gxbb and gxl also has a fractional parameter. This has
not been a problem so far because the fractional part is actually set
to 0 on these platforms, so the rate remains correct when it is ignored.
Still, it is better represent the pll the way it is, so add the frac
parameter now

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>