clk: meson-g12a: add cpu clock bindings

Add Amlogic G12A Family CPU clocks bindings, only export CPU_CLK since
it should be the only ID used.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lkml.kernel.org/r/20190304131129.7762-2-narmstrong@baylibre.com