devicetree: clocks: provide accessors for controller phandle

Provide a helper to extract the devicetree node_id for a CLOCKS
controller from a clocks phandle array.  This can be used with
DEVICE_DT_GET() to directly reference the corresponding controller
device.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Co-authored-by: Marti Bolivar <marti.bolivar@nordicsemi.no>