dt-bindings:i2c:i2c-gate: txt to yaml conversion

This file provides very little in the way of actual schema but
does provide a convenient intro to what an i2c-gate node actually
represents.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Peter Rosin <peda@axentia.se>
Cc: Wolfram Sang <wsa@the-dreams.de>
Link: https://lore.kernel.org/r/20201031181801.742585-2-jic23@kernel.org
Link: https://lore.kernel.org/r/20201128173343.390165-2-jic23@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>