i2c: designware: enable SLAVE in platform module

- Slave mode selected in platform module if the support is detected in
  the DT.

Signed-off-by: Luis Oliveira <lolivei@synopsys.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>