dt-bindings: clock: Convert ingenic,cgu.txt to YAML

Convert the ingenic,cgu.txt doc file to ingenic,cgu.yaml.

The binding documentation has been updated as well. The node can have a
child node that corresponds to the USB PHY, which happens to be present
in the middle of the CGU registers.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Rob Herring <robh@kernel.org>