Only certain system configurations may use the I2C slave mode so let the
support be optional. This allow reducing module size if needed:

   text    data     bss     dec     hex filename
  10328    1336      16   11680    2da0 drivers/i2c/busses/i2c-designware-core.ko
   7222    1136       8    8366    20ae drivers/i2c/busses/i2c-designware-core.ko
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Luis Oliveira <lolivei@synopsys.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>