iio: accel: bmc150-accel: Add DT bindings

These accelerometers have bindings used in the kernel and
several device trees but no proper bindings documentation.
Add it.

Also add a compatible for the BMA222 that I am right now
adding support for in the driver.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20201115205745.618455-1-linus.walleij@linaro.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/accel/bosch,bma255.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/accel/bosch,bma255.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Bosch BMA255 and Similar Accelerometers
+
+maintainers:
+  - Linus Walleij <linus.walleij@linaro.org>
+
+description:
+  3 axis accelerometers with varying range and I2C or SPI
+  4-wire interface.
+
+properties:
+  compatible:
+    enum:
+      - bosch,bmc150
+      - bosch,bmi055
+      - bosch,bma255
+      - bosch,bma250e
+      - bosch,bma222
+      - bosch,bma222e
+      - bosch,bma280
+
+  reg:
+    maxItems: 1
+
+  vdd-supply: true
+  vddio-supply: true
+
+  interrupts:
+    maxItems: 1
+
+  mount-matrix:
+    description: an optional 3x3 mounting rotation matrix.
+
+  spi-max-frequency:
+    maximum: 10000000
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/interrupt-controller/irq.h>
+    i2c {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        accelerometer@8 {
+            compatible = "bosch,bma222";
+            reg = <0x08>;
+            vddio-supply = <&vddio>;
+            vdd-supply = <&vdd>;
+            interrupts = <57 IRQ_TYPE_EDGE_FALLING>;
+        };
+    };
+  - |
+    # include <dt-bindings/interrupt-controller/irq.h>
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+        accel@0 {
+            compatible = "bosch,bma222";
+            reg = <0>;
+            spi-max-frequency = <10000000>;
+        };
+    };
+...