clk: meson8b: clean up cpu clocks

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

Ninja rename a5_clk to cpu_clk to better align with cpufreq convention.

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