clk: meson8b: clean up fixed rate clocks

Remove the fixed_rate registration function and helpers from clkc.[ch].
Replace unnecessary configuration struct with static initialization of
the desired clock type.

While we're here, begin the transition to a proper platform_driver and
call of_clk_add_hw_provider with a shiny new struct clk_hw_onecell_data.

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