clk: meson: g12a: add cpu clocks

Add the Amlogic G12A Family CPU Clock tree in read/only for now.

The CPU clock can either use the SYS_PLL for > 1GHz frequencies or
use a couple of div+mux from 1GHz/667MHz/24MHz source with 2 non-glitch
muxes.

Proper DVFS support will come in a second time.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
[narmstrong: fixed cpu clocks namings]
Link: https://lkml.kernel.org/r/20190304131129.7762-3-narmstrong@baylibre.com