spi: dt-bindings: convert spi-sc18is602.txt to yaml format

Convert binding doc spi-sc18is602.txt (I2C to SPI bridge) to yaml.

Additional change:
- ref spi-controller.yaml

Fix below warning:
arch/arm64/boot/dts/freescale/fsl-lx2160a-bluebox3.dtb:
/soc/i2c@2000000/i2c-mux@77/i2c@7/i2c-mux@75/i2c@0/spi@28: failed to match any schema with compatible: ['nxp,sc18is602b']

Reviewed-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Frank Li <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/nxp,sc18is.yaml

···
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/nxp,sc18is.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: NXP SC18IS602/SC18IS603 I2C to SPI bridge
+
+maintainers:
+  - Frank Li <[e-mail]>
+
+properties:
+  compatible:
+    enum:
+      - nxp,sc18is602
+      - nxp,sc18is602b
+      - nxp,sc18is603
+
+  reg:
+    maxItems: 1
+
+  clock-frequency:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    default: 7372000
+    description:
+      external oscillator clock frequency. The clock-frequency property is
+      relevant and needed only if the chip has an external oscillator
+      (SC18IS603).
+
+allOf:
+  - $ref: spi-controller.yaml#
+
+unevaluatedProperties: false
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+    i2c {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        spi@28 {
+            compatible = "nxp,sc18is603";
+            reg = <0x28>;
+            clock-frequency = <14744000>;
+        };
+    };
+

Documentation/devicetree/bindings/spi/spi-sc18is602.txt

···
-NXP SC18IS602/SCIS603
-
-Required properties:
-	- compatible : Should be one of
-		"nxp,sc18is602"
-		"nxp,sc18is602b"
-		"nxp,sc18is603"
-	- reg: I2C bus address
-
-Optional properties:
-	- clock-frequency : external oscillator clock frequency. If not
-	  specified, the SC18IS602 default frequency (7372000) will be used.
-
-The clock-frequency property is relevant and needed only if the chip has an
-external oscillator (SC18IS603).
-
-Example:
-
-	sc18is603@28 {
-		compatible = "nxp,sc18is603";
-		reg = <0x28>;
-		clock-frequency = <14744000>;
-	}