Instead of initializing and registering a struct device for each acpi_device, just
initialize and create a driverfs directory for each one. ACPI devices aren't devices,
and shouldn't be treated as such. They are firmware objects that describe devices.
They should still have representation in driverfs (and symlinks to the devices they
describe), but none of the other driver model overhead. 

In order to do this, we also add some ACPI bindings for driverfs. It gets its own top-
level directory now (though it will soon be under a more generic 'platform' directory).