bindings: input: qcom-hv-haptics: Add qcom,fifo-empty-threshold

Add optional property "qcom,fifo-empty-threshold" to specify the
FIFO empty threshold. The module would generate fifo-empty
interrupt when the number of samples in the FIFO is less than this
threshold.

Change-Id: Iceeb2d806bf6115f9fc557b656b06bb22cb77dd4