spi: dw: Add a symbols namespace for the core module

The exported from the DW SPI driver core/DMA symbols are only used by the
spi-dw-{mmio,pci,bt1}.o objects. Add these symbols to a separate
namespace then and make sure the depended modules have it imported.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20211115181917.7521-2-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Mark Brown <broonie@kernel.org>