python: Replace orderdict with collections.OrderedDict

python: Replace orderdict with collections.OrderedDict

Python 2.7 and newer has support for ordered dictionaries in the
standard library. Remove this custom class.

Change-Id: I4b720405aa3c4ce8d5c0b401eefe744a85ac3a3e
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/16362
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>