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/serial/SConscript

@@ -43,9 +43,9 @@ Import('*')
 if env['TARGET_ISA'] == 'null':
     Return()
 
-SimObject('Serial.py')
-SimObject('Terminal.py')
-SimObject('Uart.py')
+SimObject('Serial.py', sim_objects=['SerialDevice', 'SerialNullDevice'])
+SimObject('Terminal.py', sim_objects=['Terminal'], enums=['TerminalDump'])
+SimObject('Uart.py', sim_objects=['Uart', 'SimpleUart', 'Uart8250'])
 
 Source('serial.cc')
 Source('simple.cc')