leds: leds-qpnp-flash: move duration property to flash LED node

Move duration property support to flash LED node. Moving this
property is to allow camera software get flash LED duration
information, to support flash bracketing for camera sensor.

Change-Id: Id76716a89601158157437ad95317f82d12ee26d8
Signed-off-by: Jigarkumar Kishorkumar Zala <j_zala@codeaurora.org>