dt-bindings: Document the Broadcom STB wake-up timer node

Document the binding for the Broadcom STB SoCs wake-up timer node
allowing the system to generate alarms and exit low power states.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>