power: supply: core: Add psy_has_property()

Add the helper psy_has_property() to check whether a power supply
has a given property and use it instead of ad hoc iterations over
the property list in multiple locations.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>