dt-bindings: misc: lwn,bk4-spi: Add binding

Add a lwn,bk4-spi.yaml binding for Liebherr's BK4 external SPI controller.

Currently, the compatible string used for this device is "lwn,bk4",
which is the same as the board compatible string documented at fsl.yaml.

This causes several dt-schema warnings:

make dtbs_check DT_SCHEMA_FILES=fsl.yaml
...

['lwn,bk4'] is too short
'lwn,bk4' is not one of ['tq,imx8dxp-tqma8xdp-mba8xx']
'lwn,bk4' is not one of ['tq,imx8qxp-tqma8xqp-mba8xx']
'lwn,bk4' is not one of ['armadeus,imx1-apf9328', 'fsl,imx1ads']
...

Use a more specific "lwn,bk4-spi" compatible string for this
device.

Signed-off-by: Fabio Estevam <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Mark Brown <[email protected]>

Author: Fabio Estevam

Documentation/devicetree/bindings/misc/lwn,bk4-spi.yaml

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/misc/lwn,bk4-spi.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Liebherr's BK4 external SPI controller
+
+maintainers:
+  - Lukasz Majewski <[email protected]>
+
+description: |
+  Liebherr's BK4 external SPI controller is a device which handles data
+  acquisition from compatible industrial peripherals.
+  The SPI is used for data and management purposes in both master and
+  slave modes.
+
+allOf:
+  - $ref: /schemas/spi/spi-peripheral-props.yaml#
+
+properties:
+  compatible:
+    const: lwn,bk4-spi
+
+  reg:
+    maxItems: 1
+
+  spi-max-frequency:
+    maximum: 30000000
+
+  fsl,spi-cs-sck-delay: true
+
+  fsl,spi-sck-cs-delay: true
+
+required:
+  - compatible
+  - spi-max-frequency
+
+additionalProperties: false
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        spidev@0 {
+            compatible = "lwn,bk4-spi";
+            reg = <0>;
+            spi-max-frequency = <30000000>;
+            fsl,spi-cs-sck-delay = <200>;
+            fsl,spi-sck-cs-delay = <400>;
+        };
+    };