ARM: tegra: Mark Tegra30 display controller compatible with Tegra20

The display controller found on Tegra30 SoCs is backwards-compatible
with the one on Tegra20 SoCs.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>

arch/arm/boot/dts/tegra30.dtsi

@@ -141,7 +141,7 @@ gr3d {
 		};
 
 		dc@54200000 {
-			compatible = "nvidia,tegra30-dc";
+			compatible = "nvidia,tegra30-dc", "nvidia,tegra20-dc";
 			reg = <0x54200000 0x00040000>;
 			interrupts = <GIC_SPI 73 IRQ_TYPE_LEVEL_HIGH>;
 			clocks = <&tegra_car TEGRA30_CLK_DISP1>,