systemc: Keep track of progress when positionally binding.

systemc: Keep track of progress when positionally binding.

Positionally binding more than once (like with the deprecated comma or
<< operators) should pick up where it left off the last time instead
of starting again from the beginning.

Change-Id: Ifc33520d6ce40544bd0ad80a5657b1a38a7914e4
Reviewed-on: https://gem5-review.googlesource.com/c/13297
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>