greybus: interface: partition attribute group

Partition the current attribute group into four groups for UniPro,
Greybus, power and common attributes.

This is a step in refining the interface-type handling as attributes are
type dependent.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>