scons: Add sim_object and enums arguments to SimObject().

This will explicitly declare what SimObject and Enum types need to be set
up in C++, which will make importing all the SimObject modules during
the setup phase of SCons uneccessary.

Change-Id: Id2d7603daf33b236ceaa0789e2f089f589d34e62
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/49406
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/mips/SConscript

@@ -28,7 +28,8 @@
 
 Import('*')
 
-SimObject('Malta.py', tags='mips isa')
+SimObject('Malta.py', sim_objects=['MaltaCChip', 'MaltaIO', 'Malta'],
+    tags='mips isa')
 
 DebugFlag('Malta', tags='mips isa')