dtc: Remove spurious output on stderr

Outputing to stderr is best avoided unless there is an error or warning to
display. At present dtc always displays the name of the file it is compiling
and the input/output formats. For example:

DTC: dts->dts  on file "-"

This can cause problems in some build systems. For example, U-Boot shows
build errors for any boards which use dtc at present. It is typically the
only message output during such a build. The C compiler does not output
anything in general. The current dtc behaviour makes it difficult to
provide a silent build in the normal case where nothing went wrong.

Remove the message entirely.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>