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.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>