i2c: regroup documentation of bindings

Some bindings are for the bus master, some are for the slaves.
Regroup them and give them separate headings to make it clear.
Also, remove references to "generic names" which is for nodes and not
for compatibles.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>