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 <[e-mail]>
Signed-off-by: Soren Brinkmann <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/net/macb.txt

···
 - interrupts: Should contain macb interrupt
 - phy-mode: String, operation mode of the PHY interface.
   Supported values are: "mii", "rmii", "gmii", "rgmii".
+- clock-names: Tuple listing input clock names.
+	Required elements: 'pclk', 'hclk'
+	Optional elements: 'tx_clk'
+- clocks: Phandles to input clocks.
 
 Optional properties:
 - local-mac-address: 6 bytes, mac address
···
 		interrupts = <21>;
 		phy-mode = "rmii";
 		local-mac-address = [3a 0e 03 04 05 06];
+		clock-names = "pclk", "hclk", "tx_clk";
+		clocks = <&clkc 30>, <&clkc 30>, <&clkc 13>;
 	};