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

Fixes: 78b4af31 ("clk: meson-axg: add clock controller drivers")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>