systemc: "Implement" sc_interface's methods.

systemc: "Implement" sc_interface's methods.

Most don't do anything. Make default_event() return a dummy event to
avoid dereferencing a null pointer.

Change-Id: I8d6d576d3a1f585029c387cd414bbebf2d670644
Reviewed-on: https://gem5-review.googlesource.com/12072
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>