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