dt-bindings: i2c: Remove obsolete i2c.txt

Everything in i2c.txt is covered by schemas/i2c/i2c-controller.yaml in
dtschema project, so remove i2c.txt and update links to it in the tree.

Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240222174343.3482354-2-robh@kernel.org


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