power_supply: Add local extensions of string property names properly

For displaying local extensions of properties in string format,
add them between MODEL_NAME and SERIAL_NUMBER. Otherwise, those
properties would be represented as integer.

Change-Id: Iddb34d7ee6f520b789bc2e085ada4053343538f0
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>