dt-bindings: clock: samsung: add bindings for Exynos external clock

Document the bindings for Samsung Exynos external to SoC
(oscclk/XXTI/XusbXTI) clock provided on boards.  The bindings are
already implemented in most of the Exynos clock drivers and DTS files.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210825134056.219884-3-krzysztof.kozlowski@canonical.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/samsung,exynos-ext-clock.yaml

@@ -0,0 +1,46 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/clock/samsung,exynos-ext-clock.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Samsung SoC external/osc/XXTI/XusbXTI clock
+
+maintainers:
+  - Chanwoo Choi <cw00.choi@samsung.com>
+  - Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
+  - Sylwester Nawrocki <s.nawrocki@samsung.com>
+  - Tomasz Figa <tomasz.figa@gmail.com>
+
+description: |
+  Samsung SoCs require an external clock supplied through XXTI or XusbXTI pins.
+
+properties:
+  compatible:
+    enum:
+      - samsung,clock-xxti
+      - samsung,clock-xusbxti
+      - samsung,exynos5420-oscclk
+
+  "#clock-cells":
+    const: 0
+
+  clock-frequency: true
+
+  clock-output-names:
+    maxItems: 1
+
+required:
+  - compatible
+  - clock-frequency
+
+additionalProperties: false
+
+examples:
+  - |
+    fixed-rate-clocks {
+        clock {
+            compatible = "samsung,clock-xxti";
+            clock-frequency = <24000000>;
+        };
+    };