DT: hwspinlock: Add binding documentation for Spreadtrum hwspinlock

This patch adds the binding documentation for Spreadtrum hardware spinlock
device.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Baolin Wang <baolin.wang@spreadtrum.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>