power_supply: Add FG_TYPE power-supply property

Use the FG_TYPE property to report the type of FG
present in the system. For example on QG, it is
voltage-mode based QG or cc-mode based QG.

Change-Id: Iaad3ad29b2d778eed215d420a23b601587163e83
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>