ARM: dt: tegra20: Rename "gart" to "iommu"

Use functional name for DT entry instead of h/w name.
Signed-off-by: NHiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: NStephen Warren <swarren@nvidia.com>

arch/arm/boot/dts/tegra20.dtsi

@@ -429,7 +429,7 @@
 		interrupts = <0 77 0x04>;
 	};
 
-	gart {
+	iommu {
 		compatible = "nvidia,tegra20-gart";
 		reg = <0x7000f024 0x00000018	/* controller registers */
 		       0x58000000 0x02000000>;	/* GART aperture */