docs: driver-model: device: Add ATTRIBUTE_GROUPS() example

Add a paragraph about the ATTRIBUTE_GROUPS() helper macro, and use it in
the example.  Retain the old description, as it is still useful in case
of multiple groups.  Change the names of the group(s) structures, to
match the ATTRIBUTE_GROUPS() macro.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210303133845.3939403-3-geert+renesas@glider.be


Signed-off-by: Jonathan Corbet <corbet@lwn.net>