dt-bindings: Document Broadcom Cygnus SoC and clocks

Reviewed-by: Arun Parameswaran <aparames@broadcom.com>
Tested-by: Jonathan Richardson <jonathar@broadcom.com>
Reviewed-by: JD (Jiandong) Zheng <jdzheng@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>