power: supply: core: Add a macro that maps enum properties to text values

Reduce the number of touch points to add a new enum property to the
power_supply class by mapping the array of text values to the device
attribute descriptor. A new enum property can now added by creating an
array with the text values named POWER_SUPPLY_${PROPNAME}_TEXT and
adding POWER_SUPPLY_ENUM_ATTR(${PROPNAME}) to the power_supply_attrs
array.

Signed-off-by: Mathew King <mathewk@chromium.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>