power_supply: add flash_current_max as a property

Add POWER_SUPPLY_PROP_FLASH_CURRENT_MAX as a power supply property in
order to report how much current can be drawn out of the battery
before the battery hits the VIN threshold.

Change-Id: I771f64d4b903117824907607af6529a86208bf32
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>