dt-bindings: soc: samsung: exynos-pmu: add exynos7870-pmu compatible

dt-bindings: soc: samsung: exynos-pmu: add exynos7870-pmu compatible

Document the compatible string for the Exynos7870 PMU. It's compatible
with the Exynos7 PMU design. It handles syscon reboot, syscon reboot mode,
as well as other system control registers (i.e registers for the USB PHY).

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Link: https://lore.kernel.org/r/20250301-exynos7870-v4-1-2925537f9b2a@disroot.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>