Documentation: devicetree: Document BCM23550 bindings

Add binding document for Broadcom BCM23550 SoC.

BCM23550 has a Cluster Dormant Control IP block that holds cores
in an idle state. Introduce a new CPU enable method in which the CDC is
accessed to bring the core online.

Signed-off-by: Chris Brand <chris.brand@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>