dt-bindings: pinctrl: samsung: Add Exynos850 doc

Document compatible string for Exynos850 SoC. Nothing else is changed,
as Exynos850 SoC uses already existing samsung pinctrl driver.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20210811114827.27322-2-semen.protsenko@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>