scripts/dts/extract: Generate unique defines based on compat & instance

Add support to generate defines of the form
DT_<COMPAT>_<INSTANCE>_<PROP>.  The idea is that we can utilize this in
drivers to remove the need for dts_fixup.h.  The <INSTANCE> value is
determined by the script and starts at 0 and counts up for each instance
of a given <COMPAT>.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>