dt-bindings: leds-qpnp-flash-v2: Add support to control ramp time

Add device tree properties "qcom,ramp-up-step" and
"qcom,ramp-down-step", to configure the time steps for
flash current ramp up and ramp down.

Change-Id: I0cfe8c21f55fe864364337bb6c909426be966d6d