dt-bindings: clock: add the Amlogic Meson8 DDR clock controller binding

Amlogic Meson8, Meson8b and Meson8m2 SoCs have a DDR clock controller in
the MMCBUS registers. There is no public documentation on this, but the
GPL u-boot sources from the Amlogic BSP show that:
- it uses the same XTAL input as the main clock controller
- it contains a PLL which seems to be implemented just like the other
  PLLs in this SoC
- there is a power-of-two PLL post-divider

Add the documentation and header file for this DDR clock controller.
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>

Documentation/devicetree/bindings/clock/amlogic,meson8-ddr-clkc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/clock/amlogic,meson8-ddr-clkc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Amlogic DDR Clock Controller Device Tree Bindings
+
+maintainers:
+  - Martin Blumenstingl <martin.blumenstingl@googlemail.com>
+
+properties:
+  compatible:
+    enum:
+      - amlogic,meson8-ddr-clkc
+      - amlogic,meson8b-ddr-clkc
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    items:
+      - const: xtal
+
+  "#clock-cells":
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - clock-names
+  - "#clock-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    ddr_clkc: clock-controller@400 {
+      compatible = "amlogic,meson8-ddr-clkc";
+      reg = <0x400 0x20>;
+      clocks = <&xtal>;
+      clock-names = "xtal";
+      #clock-cells = <1>;
+    };
+
+...

include/dt-bindings/clock/meson8-ddr-clkc.h

+/* SPDX-License-Identifier: GPL-2.0 */
+
+#define DDR_CLKID_DDR_PLL_DCO			0
+#define DDR_CLKID_DDR_PLL			1