spi: dw: Make DMA request line assignments explicit for Intel Medfield

The 2afccbd2 ("spi: dw: Discard static DW DMA slave structures")
did a clean up of global variables, which is fine, but messed up with
the carefully provided information in the custom DMA slave structures.
There reader can find an assignment of the DMA request lines in use.

Partially revert the above mentioned commit to restore readability
and maintainability of the code.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200529183150.44149-1-andriy.shevchenko@linux.intel.com


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