devicetree: macb: Document clock properties

The macb driver uses the clock bindings. Document the required
properties, especially the driver specific clock-names.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Rob Herring <robh@kernel.org>