dt-bindings: power: reset: convert nvmem-reboot-mode bindings to YAML

Convert the DT binding document for nvmem-reboot-mode from .txt to YAML.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>