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: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>