Platform devices are devices commonly found on the motherboard of systems. This
includes legacy devices (serial ports, floppy controllers, parallel ports, etc)
and host bridges to peripheral buses. 

We already had a platform bus type, which gives a way to group platform devices
and drivers, and allow each to be bound to each other dynamically. Though before,
it didn't do anything. It still doesn't do much, but we now have:

- struct platform_device, which generically describes platform deviecs. This only
  includes a name and id in addition to a struct device, but more may be added later.

- implelemnt platform_device_register() and platform_device_unregister() to handle
  adding and removing these devices. 

- Create legacy_bus - a default parent device for legacy devices. 

- Change the floppy driver to define a platform_device (instead of a sys_device). 
  In driverfs, this gives us now:

a# tree -d /sys/bus/platform/
/sys/bus/platform/
|-- devices
|   `-- floppy0 -> ../../../root/legacy/floppy0
`-- drivers

and

# tree -d /sys/root/legacy/
/sys/root/legacy/
`-- floppy0