of/platform: Initialise dev->fwnode appropriately

Whilst we're some of the way towards a universal firmware property
interface, drivers which deal with both OF and ACPI probing end up
having to do things like this:

    dev->of_node ? &dev->of_node->fwnode : dev->fwnode

This seems unnecessary, when the OF code could instead simply fill in
the device's fwnode when binding the of_node, and let the drivers use
dev->fwnode either way. Let's give it a go and see what falls out.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>