Add support for Gem5 for arm64

Add Yocto support for Gem5:
- add kernel configuration
- add recipes to build an image using boot-wrapper-aarch64
- add recipes to build Gem5 native to run the produced image
- add script to start image using gem5
- add recipe to create a dtb using gem5
- add recipe to build an image using gem5 boot wrapper
- add dts to linux kernel for gem5 for arm64
- add scons for python2 as this is required to build gem5 (until gem5
will support python3)
- add xen patch to limit number of interrupts if hardware is claiming to
have to more interrupts then the number supported by xen

When an image has been built it can be directly started using gem5 using
this command (console is available using telnet localhost 3456):
./tmp/deploy/tools/start-gem5.sh

Change-Id: Iec5ef735fc22643685e29ffbdaf63a55b1111c60
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>