regulator: userspace: use sysfs_create_group

and avoid introducing our own loops for creating
several sysfs entries.

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>