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/Uart.py

@@ -33,6 +33,7 @@ from Device import BasicPioDevice
 class Uart(BasicPioDevice):
     type = 'Uart'
     abstract = True
+    platform = Param.Platform(Parent.any, "Platform this device is part of.")
     terminal = Param.Terminal(Parent.any, "The terminal")
 
 class Uart8250(Uart):