arm/tegra: Make MACH_TEGRA_DT depend on ARCH_TEGRA_2x_SOC

Now that Tegra20 and Tegra30 device tree board files are separate,
MACH_TEGRA_DT (which enables the Tegra20 device tree board file) should
depend on Tegra20 support being enabled.
Signed-off-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NOlof Johansson <olof@lixom.net>

arch/arm/mach-tegra/Kconfig

@@ -66,6 +66,7 @@ config MACH_SEABOARD
 
 config MACH_TEGRA_DT
 	bool "Generic Tegra20 board (FDT support)"
+	depends on ARCH_TEGRA_2x_SOC
 	select USE_OF
 	help
 	  Support for generic NVIDIA Tegra20 boards using Flattened Device Tree