dt-bindings: power: reset: add document for reboot-mode driver

Add device tree bindings document for reboot-mode driver.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Git-commit: 376e27e485cbe9f903369a002d07d5a01225d1d3
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/


Change-Id: Ic0f2a51b5af661fd23576ae5135c920b92f0cdfd
Signed-off-by: Xiaogang Cui <xiaogang@codeaurora.org>