clk: meson: axg: add the fractional part of the fixed_pll
The fixed_pll also has a fractional part. On axg s400 board, without
this parameter, the calculated rate is off by ~8Mhz (0,4%). The fixed_pll
being the root of the peripheral clock tree, this error is propagated to
the rest of the clocks
Adding the definition of the parameter fixes the problem