power: supply: core: Cleanup power supply sysfs attribute list

Make the device attribute list used to create sysfs attributes more
robust by decoupling the list order from order of the enum defined in
power_supply.h. This is done by using a designated initializer in the
POWER_SUPPLY_ATTR macro.
Signed-off-by: Mathew King <mathewk@chromium.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>