dt-bindings: i3c: Convert the bus description to yaml

Attempting a conversion of the i3c.txt file to yaml schema with
minimal content changes.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210121101808.14654-2-miquel.raynal@bootlin.com