dt-bindings: clock: Convert i.MX21 clock to json-schema

Convert the i.MX21 clock binding to DT schema format using json-schema,
can NOT find any CCM interrupt info from reference manual and DT file,
so interrupts property is removed from original binding doc.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/clock/imx21-clock.txt

-* Clock bindings for Freescale i.MX21
-
-Required properties:
-- compatible  : Should be "fsl,imx21-ccm".
-- reg         : Address and length of the register set.
-- interrupts  : Should contain CCM interrupt.
-- #clock-cells: Should be <1>.
-
-The clock consumer should specify the desired clock by having the clock
-ID in its "clocks" phandle cell. See include/dt-bindings/clock/imx21-clock.h
-for the full list of i.MX21 clock IDs.
-
-Examples:
-	clks: ccm@10027000{
-		compatible = "fsl,imx21-ccm";
-		reg = <0x10027000 0x800>;
-		#clock-cells = <1>;
-	};
-
-	uart1: serial@1000a000 {
-		compatible = "fsl,imx21-uart";
-		reg = <0x1000a000 0x1000>;
-		interrupts = <20>;
-		clocks = <&clks IMX21_CLK_UART1_IPG_GATE>,
-			 <&clks IMX21_CLK_PER1>;
-		clock-names = "ipg", "per";
-	};

Documentation/devicetree/bindings/clock/imx21-clock.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/clock/imx21-clock.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Clock bindings for Freescale i.MX21
+
+maintainers:
+  - Alexander Shiyan <shc_work@mail.ru>
+
+description: |
+  The clock consumer should specify the desired clock by having the clock
+  ID in its "clocks" phandle cell. See include/dt-bindings/clock/imx21-clock.h
+  for the full list of i.MX21 clock IDs.
+
+properties:
+  compatible:
+    const: fsl,imx21-ccm
+
+  reg:
+    maxItems: 1
+
+  '#clock-cells':
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - '#clock-cells'
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/imx21-clock.h>
+
+    clock-controller@10027000 {
+        compatible = "fsl,imx21-ccm";
+        reg = <0x10027000 0x800>;
+        #clock-cells = <1>;
+    };
+
+    serial@1000a000 {
+        compatible = "fsl,imx21-uart";
+        reg = <0x1000a000 0x1000>;
+        interrupts = <20>;
+        clocks = <&clks IMX21_CLK_UART1_IPG_GATE>,
+                 <&clks IMX21_CLK_PER1>;
+        clock-names = "ipg", "per";
+    };