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/alpha/AlphaBackdoor.py

@@ -36,5 +36,6 @@ class AlphaBackdoor(BasicPioDevice):
     cpu = Param.BaseCPU(Parent.cpu[0], "Processor")
     disk = Param.SimpleDisk("Simple Disk")
     terminal = Param.Terminal(Parent.any, "The console terminal")
+    platform = Param.Platform(Parent.any, "Platform this device is part of.")
     if buildEnv['FULL_SYSTEM']: # No AlphaSystem in SE mode.
         system = Param.AlphaSystem(Parent.any, "system object")