ARM: dt: tegra: rename board files to match SoC

Most ARM ${board}.dts files are already named ${soc}-${board}.dts. This
change modifies the Tegra board files to be named the same way for
consistency.

Once a related change is made in U-Boot, this will cause both U-Boot and
the kernel to use the same names for the .dts files and SoC identifiers,
thus allowing U-Boot's recently added "soc" and "board" environment
variables to be used to construct the name of Tegra .dtb files, and hence
allow board-generic U-Boot bootcmd scripts to be written.

Signed-off-by: Stephen Warren <swarren@nvidia.com>