dev, arm: add a9mpcore global timer device

Change-Id: I6d8a5e3795291b2a4cce022f555cf4b04f997538
Signed-off-by: Gedare Bloom <gedare@rtems.org>
Reviewed-on: https://gem5-review.googlesource.com/3262
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>