Devices have inter-dependencies some times. For example a device that
needs to run at 800 MHz, needs another device (e.g. Its power domain) to
be configured at a particular operating performance point.
This patch introduces a new property "required-opp" which can be present
directly in a device's node (if it doesn't need to change its OPPs), or
in device's OPP nodes. More details on the property can be seen in the
binding itself.