Travis-CI: Switch back to using the top of tree dtc

In a0f3e3df we switched to using the Ubuntu-provided dtc as travis
was having a problem with the number of warnings that were generated by
the newer dtc.  This is no longer a concern as we now have the same
logic as Linux to enable/disable additional more stringent warnings.  Go
back to building dtc from source.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested on travis-ci:
Tested-by: Simon Glass <sjg@chromium.org>