dt-bindings: soc: samsung: exynos-pmu: add reboot-mode

ExynosAutov9 gained a reboot-mode node, so document the property to fix
warning:

  exynosautov9-sadk.dtb: system-controller@10460000: 'reboot-mode' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220706160257.27579-3-krzysztof.kozlowski@linaro.org