dts: stmmac: Document the clocks property in the stmmac base document

Add the clocks and clock-names property to the base stmmac dts bindings
document.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

diff --cc Documentation/devicetree/bindings/net/stmmac.txt
Simple merge