scripts/dtc: dtx_diff - make help text formatting consistent

None of the help texts use capitalization, except the one for the -T
option.  Drop the capitalization for consistency.
Split the single long line that doesn't fit in 80 characters.

Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200819124709.20401-1-geert+renesas@glider.be