clk: meson: remove obsolete comments

Over time things changes in CCF and issues have been fixed in meson
controllers.

Now, clk81 is decently modeled by read-only PLLs, a mux, a divider
and a gate. We can remove the FIXME comments related to clk81.
Also remove the comment about devm_clk_hw_register, as there is
apparently nothing wrong with it.

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