iio:imu:mpu6050 update i2c bindings to reflect i2c-gate and supported parts

These bindings are somewhat lagging the state of the driver.  The i2c-gate
bindings are documented elsewhere, but it seems sensible to at least have
an example and cross reference in here.  SPI bindings will need to be the
subject of a future patch.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>