clk: meson8b: clean up composite clocks

Remove the composite clock registration function and helpers. Replace
unnecessary configuration struct with static initialization of the
desired clock type.

To preserve git bisect this patch also flips the switch and starts using
of_clk_add_hw_provider instead of the deprecated meson_clk_register_clks
method. As a byproduct clk.c can be deleted.

Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>