spi/pxa2xx: break out the private DMA API usage into a separate file

The PXA SPI driver uses PXA platform specific private DMA implementation
which does not work on non-PXA platforms. In order to use this driver on
other platforms we break out the private DMA implementation into a separate
file that gets compiled only when CONFIG_SPI_PXA2XX_PXADMA is set. The DMA
functions are stubbed out if there is no DMA implementation selected (i.e
we are building on non-PXA platform).

While we are there we can kill the dummy DMA bits in pxa2xx_spi.h as they
are not needed anymore for CE4100.

Once this is done we can add the generic DMA engine support to the driver
that allows usage of any DMA controller that implements DMA engine API.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Lu Cao <lucao@marvell.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/spi/Kconfig

@@ -297,6 +297,12 @@ config SPI_PPC4xx
 	help
 	  This selects a driver for the PPC4xx SPI Controller.
 
+config SPI_PXA2XX_PXADMA
+	bool "PXA2xx SSP legacy PXA DMA API support"
+	depends on SPI_PXA2XX && ARCH_PXA
+	help
+	  Enable PXA private legacy DMA API support.
+
 config SPI_PXA2XX
 	tristate "PXA2xx SSP SPI master"
 	depends on ARCH_PXA || PCI