python: Add support for multiplying proxies to compatible Param

python: Add support for multiplying proxies to compatible Param

Previously we allowed multiplications between proxy Param and
compatible constants (int, long, float). This change extends this
functionality and adds support for multiplying with between proxy
Param and compatible proxy Param.

Change-Id: I23a083881ae4d770e818895b893534767cd2472d
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11510
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>