dts/extract_dts_includes.py: Fix IRQ define name creation

We were missing the cell name in how we create the IRQ define.  This was
working fine because the only name we had been using with IRQ which we
would shorten down.

Change-Id: I8449c8ced1a9284982d3d5b07c6acdcf26e965e2
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>