dt-bindings: power: reset: restart-handler: add common schema

Add common schema for restart and shutdown handlers, so they all use
same meaning of "priority" field.  The Linux drivers already have this
property and some systems want to customize it per-board in DTS.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>