dt-bindings: reset: syscon-reboot: Add priority property

This new optional priority property allows to specify custom priority level
of reset device. Prior this change priority level was hardcoded to 192 and
not possible to specify or change. Specifying other value is needed for
some boards. Default level when not specified stays at 192 as before.

Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
[add $ref to existing allOf to fix duplication warning]
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>