dt-bindings: power: reset: Add qcom pon binding

The Power On device for Qcom PM 8xxx is a MFD supporting pwrkey and
resin along with the Android reboot-mode.

Add the binding describing this.

Suggested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>