Documentation: devicetree: move trivial-devices out of I2C realm

The I2C directory is for I2C master controllers, not for clients. So,
move the file describing trivial devices one layer upwards where it
fits its generic nature way better.
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Rob Herring <robh@kernel.org>