i2c: designware-pci: Add a note about struct dw_scl_sda_cfg usage

Add a note about struct dw_scl_sda_cfg usage to discourage people
of using this structure on new platforms. Instead they should try
hard to put the needed information into firmware descriptions.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>