arm64: dts: freescale: Rename DTB overlay source files from .dts to .dtso

DTB Overlays (.dtbo) can now be built from source files with the
extension (.dtso). This makes it clear what is the content of the files
and differentiates them from base DTB source files.

Convert the DTB overlay source files in the arm64/freescale directory.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>