clk: meson: gxbb: protect against holes in the onecell_data array

The clock controller is getting more complex and it might be possible, in
the future, to have holes in the clk_hw_onecell_data array. Just make sure
we skip those holes if it ever happens.

Acked-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>