dt-bindings: samsung: exynos-chipid: Document Exynos850 compatible

Add compatible string for Exynos850 chip-id. While at it, use enum
instead of items/const, to reduce further cluttering of "compatible"
list.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20211014133508.1210-2-semen.protsenko@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>