dt-bindings: serial: brcm,bcm6345-uart: convert to the json-schema

This helps validating DTS files.

Redundant generic descriptions (e.g. "The base address of the UART
register bank") were dropped and periph_clk was dropped from example as
not required.
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20210829131927.13682-1-zajec5@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/brcm,bcm6345-uart.txt

-* BCM63xx UART
-
-Required properties:
-
-- compatible: "brcm,bcm6345-uart"
-
-- reg: The base address of the UART register bank.
-
-- interrupts: A single interrupt specifier.
-
-- clocks: Clock driving the hardware; used to figure out the baud rate
-  divisor.
-
-
-Optional properties:
-
-- clock-names: Should be "refclk".
-
-Example:
-
-	uart0: serial@14e00520 {
-		compatible = "brcm,bcm6345-uart";
-		reg = <0x14e00520 0x18>;
-		interrupt-parent = <&periph_intc>;
-		interrupts = <2>;
-		clocks = <&periph_clk>;
-		clock-names = "refclk";
-	};
-
-	clocks {
-		periph_clk: periph_clk@0 {
-			compatible = "fixed-clock";
-			#clock-cells = <0>;
-			clock-frequency = <54000000>;
-		};
-	};

Documentation/devicetree/bindings/serial/brcm,bcm6345-uart.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/serial/brcm,bcm6345-uart.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: BCM63xx UART
+
+maintainers:
+  - Rafał Miłecki <rafal@milecki.pl>
+
+allOf:
+  - $ref: serial.yaml#
+
+properties:
+  compatible:
+    const: brcm,bcm6345-uart
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    maxItems: 1
+
+  clock-names:
+    const: refclk
+
+unevaluatedProperties: false
+
+required:
+  - reg
+  - interrupts
+  - clocks
+
+examples:
+  - |
+    serial@14e00520 {
+        compatible = "brcm,bcm6345-uart";
+        reg = <0x14e00520 0x18>;
+        interrupt-parent = <&periph_intc>;
+        interrupts = <2>;
+        clocks = <&periph_clk>;
+        clock-names = "refclk";
+    };