learning_gem5: Add code for simple SimObject

This adds code from Learning gem5 Part II.

See http://learning.gem5.org/book/part2/helloobject.html

Change-Id: Ic2caa07876ca57f937729c27ce29b2cd8bf2380c
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/5020
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>