i2c: designware: Allow slave mode for PCI enumerated devices

Allow slave mode for PCI enumerated devices by calling a common i2c_dw_probe()
instead of i2c_dw_probe_master().

While dropping dependency to platform driver in slave module, move its
configuration section above, closer to core.

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