dt-bindings: clock: add the Amlogic Meson8 DDR clock controller binding

Amlogic Meson8, Meson8b and Meson8m2 SoCs have a DDR clock controller in
the MMCBUS registers. There is no public documentation on this, but the
GPL u-boot sources from the Amlogic BSP show that:
- it uses the same XTAL input as the main clock controller
- it contains a PLL which seems to be implemented just like the other
  PLLs in this SoC
- there is a power-of-two PLL post-divider

Add the documentation and header file for this DDR clock controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>