clk: meson: add gp0 frac parameter for axg and gxl

Add the frac parameter for the gp0 pll of the axg and gxl.
This allows to achieve rates between the fixed settings provided
by the table.

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