fastmodel: create base class for EVS CPU

fastmodel: create base class for EVS CPU

Previously we use attribute and event for communication between gem5
SimObject to systemC fastmodel sc_module. Creating a base class allows us
to perform casting once and get all the interface required. Also,
instead of warning on attribute not found, we should make simulator
panic if the sc_module does not provide the interface we need.

Change-Id: I91e1036cb792d556dfc4010e7a0f138b1519b079
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40277
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>