spi: dw-pci: describe Intel MID controllers better

There are more that one SPI controller on the Intel MID boards. This patch
describes the status and IDs of them. From now on we also have to care about
bus number that must be unique per host.

According to the specification the SPI1 has 5 bits for chip selects and SPI2
only 2 bits. The patch makes it depend to PCI ID.

The first controller (SPI1) is DMA capable, meanwhile SPI2 can share same
channels (via software switch) such functionality is not in the scope of this
patch. Thus, attempt to init DMA for SPI2 will always fail for now.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>