python: Fix params/proxy import loop

python: Fix params/proxy import loop

There is a circular dependency between params and proxy at import
time. This causes issues for Python 3. Add the imports to the specific
methods with the dependencies to make the import happen when the
method is executed instead.

Change-Id: I770112fd3c07c395459e204976942bda3dc7236f
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15993
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Juha Jäykkä <juha.jaykka@arm.com>