dt-bindings: pinctrl: samsung: add exynos7870-pinctrl compatible

Document a dedicated compatible string for Exynos7870's pin controllers,
having 8 GPIO pin banks.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Link: https://lore.kernel.org/r/20250301-exynos7870-pinctrl-v3-1-ba1da9d3cd2f@disroot.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>