SE/FS: Put platform pointers in fewer objects.

Not all objects need a platform pointer, and having one creates a dependence
on their being a platform object. This change removes the platform pointer to
from the base device object and moves it into subclasses that actually need
it.

src/dev/Device.py

@@ -34,7 +34,6 @@ class PioDevice(MemObject):
     type = 'PioDevice'
     abstract = True
     pio = Port("Programmed I/O port")
-    platform = Param.Platform(Parent.any, "Platform this device is part of")
     system = Param.System(Parent.any, "System this device is part of")
 
 class BasicPioDevice(PioDevice):