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

Documentation/devicetree/bindings/arm/samsung/exynos-chipid.yaml

@@ -11,8 +11,9 @@ maintainers:
 
 properties:
   compatible:
-    items:
-      - const: samsung,exynos4210-chipid
+    enum:
+      - samsung,exynos4210-chipid
+      - samsung,exynos850-chipid
 
   reg:
     maxItems: 1