spi: Rework DMA mapped flag

spi: Rework DMA mapped flag

Merge series from Andy Shevchenko <andriy.shevchenko@linux.intel.com>:

The first part of the series (patches 1 to 7) is an introduction
of a new helper followed by the user conversion.

This consolidates the same code and also makes patch 8 (last one)
be localised to the SPI core part.

The last patch is the main rework to get rid of a recently introduced
hack with a dummy SG list and move to the transfer-based DMA mapped
flag.

That said, the patches 1 to 7 may be applied right away since they
have no functional change intended, while the last one needs more
testing and reviewing.