i2c: document binding for multi-master case

We need this binding because some I2C master drivers will need to adapt
their PM settings for the arbitration circuitry.

Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>