dt-bindings: arm: Document Broadcom SoCs 'secondary-boot-reg'

Consolidate and move the 'secondary-boot-reg' property from the 3
existing binding documents into the main cpus.yaml documentation, also
make sure that the property is enforced when relevant.

Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>