clk: meson: remove obsolete cpu_clk

meson8b cpu_clk has been replaced by a set of divider and mux clocks.
meson_cpu_clk is no longer used and can be removed

Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>