dt-bindings: qpnp-fg-gen4: update qcom,fg-batt-temp-hyst description

Battery temperature hysteresis can be configured without the
dependency of cold and hot thresholds. Hence update the
description of qcom,fg-batt-temp-hyst property accordingly.

Change-Id: Ib7a65eb70a6e93dc7b7eb3284d5d9ac61a1ece53
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>