scripts/dtc: dtx_diff - Show real file names in diff header

As the comparison uses process substitution to pass files after
conversion to DTS format, the diff header doesn't show the real
filenames, but the names of the file descriptors used:

    --- /dev/fd/63  2017-06-22 11:21:47.531637188 +0200
    +++ /dev/fd/62  2017-06-22 11:21:47.531637188 +0200

This is especially annoying when comparing a bunch of DT files in a
loop, as the output doesn't show a clue about which files it refers to.

Fix this by explicitly passing the original file names to the diff
command using the --label option, giving e.g.:

    --- arch/arm/boot/dts/r8a7791-koelsch.dtb
    +++ arch/arm/boot/dts/r8a7791-porter.dtb

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>