iio: gyro: Add MPU-3050 device tree bindings

This adds device tree bindings for the MPU-3050 gyroscope. Since it
is the first set of bindings for a gyroscope, the folder for it
is also created.

Cc: devicetree@vger.kernel.org
Cc: Peter Rosin <peda@axentia.se>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>