dts/extract_dts_includes.py: Replace cell_prefix to cell_string

Only the IRQ prasing was using cell_prefix and cell_string does the same
thing so lets just use that everywhere.

Change-Id: I2ef9b1e2c1baef54b71d811c835b29a9ca8fe81b
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>