power_supply: add POWER_SUPPLY_PROP_FLASH_TRIGGER

Add the power_supply property FLASH_TRIGGER in order to run algorithms
and workarounds for when the flash led need to strobe.

Change-Id: Icb2b25ffaaba329132f095f2377a22078633bf63
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>