systemc: Add bind() to sc_port_base.

systemc: Add bind() to sc_port_base.

sc_port_base is supposed to be implementation defined, but internal
details of it are relied on by the systemc tests.

Change-Id: I53d84e708a5543a2cf4bd0deffc2efea1c008d97
Reviewed-on: https://gem5-review.googlesource.com/11350
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>