spi: rb4xx: add corresponding device tree documentation

This patch adds the correcsponding MikroTik vendor and device tree
documentation
Signed-off-by: Christopher Hill <ch6574@gmail.com>
Link: https://lore.kernel.org/r/20200521183631.37806-3-ch6574@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/mikrotik,rb4xx-spi.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/spi/mikrotik,rb4xx-spi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MikroTik RB4xx series SPI master
+
+maintainers:
+  - Gabor Juhos <juhosg@openwrt.org>
+  - Bert Vermeulen <bert@biot.com>
+
+allOf:
+  - $ref: "spi-controller.yaml#"
+
+properties:
+  compatible:
+    const: mikrotik,rb4xx-spi
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+examples:
+  - |
+    spi: spi@1f000000 {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        compatible = "mikrotik,rb4xx-spi";
+        reg = <0x1f000000 0x10>;
+    };
+
+...
\ No newline at end of file

Documentation/devicetree/bindings/vendor-prefixes.yaml

@@ -633,6 +633,8 @@ patternProperties:
     description: Microsoft Corporation
   "^mikroe,.*":
     description: MikroElektronika d.o.o.
+  "^mikrotik,.*":
+    description: MikroTik
   "^miniand,.*":
     description: Miniand Tech
   "^minix,.*":