wm831x_backup: Support multiple instances

If there are multiple wm831x devices in the system we need to assign
different names to the power supply devices in order to ensure we can
create the sysfs entries for them.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>