i2c: designware: slave: Set DW I2C core module dependency

DW APB I2C slave code in fact depends on the DW I2C driver core, but not
on the platform code as it used to be before commit 90bc1ee6de9f ("i2c:
designware: Allow slave mode for PCI enumerated devices"). Yes, the I2C
slave interface is currently supported by both the platform and PCI
versions of the IP core, but it still depends on the DW I2C core
functionality and must be available only if the last one is enabled.
So make sure the DW APB I2C slave config is only available if the
I2C_DESIGNWARE_CORE config is enabled.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>