dev-arm: Factory SimObject for generating ArmInterruptPin

dev-arm: Factory SimObject for generating ArmInterruptPin

With this patch the python ArmInterruptPin SimObject matches to the
C++ ArmInterruptPinGen. The latter is in charge of generating
the ArmInterruptPin (which is not a SimObject anymore).
This is meant to ease the generation of ArmInterruptPins: by
not being SimObjects we are not forced to instantiate them
in the configuration script; we can generate them dynamically
instead throughout simulation.

Change-Id: I917d73a26168447221f5993c8ae975ee3771e3bf
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/12401
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>