power_supply: Add "unknown" in power supply type

For the default value of power supply type, "unknown" is added.
With default prop value, supply type property can be displayed
as default - "Unknown".

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>