dt-bindings: auxdisplay: Add Maxim MAX6958/6959

Add initial device tree documentation for Maxim MAX6958/6959.
As per reviewer's request mention the fact of absence the reset
and power enable pins, since the hardware is quite simple.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>

Documentation/devicetree/bindings/auxdisplay/maxim,max6959.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/auxdisplay/maxim,max6959.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MAX6958/6959 7-segment LED display controller
+
+maintainers:
+  - Andy Shevchenko <andriy.shevchenko@linux.intel.com>
+
+description:
+  The Maxim MAX6958/6959 7-segment LED display controller provides
+  an I2C interface to up to four 7-segment LED digits. The MAX6959,
+  in comparison to MAX6958, adds input support. Type of the chip can
+  be autodetected via specific register read, and hence the features
+  may be enabled in the driver at run-time, in case they are requested
+  via Device Tree. A given hardware is simple and does not provide
+  any additional pins, such as reset or power enable.
+
+properties:
+  compatible:
+    const: maxim,max6959
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    i2c {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        display-controller@38 {
+            compatible = "maxim,max6959";
+            reg = <0x38>;
+        };
+    };