Documentation: i2c: Add doc for I2C sysfs

This doc helps Linux users navigate through I2C sysfs and learn
the system I2C topology.
Signed-off-by: Alex Qiu <xqiu@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wolfram Sang <wsa@kernel.org>