power_supply: Add core support for supplied_from

This patch adds support for supplies to register a list of char *'s which
represent the list of supplies which supply them. This is the opposite as
the supplied_to list.

This change maintains support for supplied_to until all drivers which make
use of it already are converted.

Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>