powerpc: vio_cmo: use dev_groups and not dev_attrs for bus_type

The dev_attrs field has long been "depreciated" and is finally being
removed, so move the driver to use the "correct" dev_groups field
instead for struct bus_type.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Johan Hovold <johan@kernel.org>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: <linuxppc-dev@lists.ozlabs.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>