leds: qpnp-flash-v2: Fix property names in DT bindings documentation

There is a mismatch for the following property names between
driver and DT bindings documentation.

- qcom,vph-droop-threshold-mv
- qcom,vph-droop-hysteresis-mv

Fix them.

Change-Id: I08da6127be24df666c9c9ced871c34476d68fb32
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>