power: supply: sysfs: parse string as enum when writing property

This fixes the TODO to parse strings and convert them to enum values
when writing to a power_supply class property sysfs attribute.

There is at least one driver that has a writable enum property that
previously could only be written as an integer, so a fallback to writing
enums as integers instead of strings is provided so we don't break existing
userspace programs.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>