rpmsg: 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: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: <linux-remoteproc@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>