scripts/dts/extract_dts_includes.py: allow multiple fixup files
scripts/dts/extract_dts_includes.py: allow multiple fixup files

Allow the script to take multiple -f (fixup) file options.  We output
the fixup files in order that the -f options are passed.  This will
allow us to have a common soc fixup and board fixup if we desire.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

galak/zephyr,explora26/zephyr,Vudentz/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,finikorg/zephyr,punitvara/zephyr,ldts/zephyr,aceofall/zephyr-iotos,explora26/zephyr,kraj/zephyr,kraj/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,ldts/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,mbolivar/zephyr,nashif/zephyr,nashif/zephyr,explora26/zephyr,zephyriot/zephyr,fbsder/zephyr,punitvara/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,Vudentz/zephyr,kraj/zephyr,ldts/zephyr,kraj/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,mbolivar/zephyr,fbsder/zephyr,zephyriot/zephyr,Vudentz/zephyr,fbsder/zephyr,mbolivar/zephyr,fbsder/zephyr,mbolivar/zephyr,finikorg/zephyr,kraj/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr