cpu, sim: Return PortProxy &s from all the proxy accessors.

This is a step towards merging the accessors for SE and FS modes.

Change-Id: I76818ab88b97097ac363e243be9cc1911b283090
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18579
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/cpu/thread_state.cc

@@ -133,7 +133,7 @@ ThreadState::getPhysProxy()
     return *physProxy;
 }
 
-FSTranslatingPortProxy &
+PortProxy &
 ThreadState::getVirtProxy()
 {
     assert(FullSystem);
@@ -141,7 +141,7 @@ ThreadState::getVirtProxy()
     return *virtProxy;
 }
 
-SETranslatingPortProxy &
+PortProxy &
 ThreadState::getMemProxy()
 {
     assert(!FullSystem);