dt: bindings: tegra20-emc: Document clock property

Embedded memory controller has a corresponding clock, document the clock
property.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/arm/tegra/nvidia,tegra20-emc.txt

@@ -11,6 +11,7 @@ Properties:
   set of tables can be present and said tables will be used
   irrespective of ram-code configuration.
 - interrupts : Should contain EMC General interrupt.
+- clocks : Should contain EMC clock.
 
 Child device nodes describe the memory settings for different configurations and clock rates.
 
@@ -22,6 +23,7 @@ Example:
 		compatible = "nvidia,tegra20-emc";
 		reg = <0x7000f4000 0x200>;
 		interrupts = <0 78 0x04>;
+		clocks = <&tegra_car TEGRA20_CLK_EMC>;
 	}