power: power_supply: add POWER_SUPPLY_PROP_FLASH_ACTIVE property

Add a local extension property POWER_SUPPLY_PROP_FLASH_ACTIVE
to the list of power supply properties. This will be used to
notify the power supply if high current loads like flash
is active or not.

CRs-Fixed: 793295
Change-Id: I02f27da81f13a258b8c7cd72dae515698a38a3e5
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>