dt-bindings:iio:accel:bosch,bma220 device tree binding documentation

Linux driver currently relies on the driver.name to match to the
compatible, but that doesn't mean it isn't a good idea to document it.

Only real complexity is the 3 separate power supplies.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210401174112.320497-3-jic23@kernel.org