spi: spi-imx: add DMA support

Enable DMA support on i.mx6. The read speed can increase from 600KB/s
to 1.2MB/s on i.mx6q. You can disable or enable dma function in dts.
If not set "dma-names" in dts, spi will use PIO mode. This patch only
validate on i.mx6, not i.mx5, but encourage ones to apply this patch
on i.mx5 since they share the same IP.

Note:
  Sometime, there is a weid data in rxfifo after one full tx/rx
transfer finish by DMA on i.mx6dl, so we disable dma functhion on
i.mx6dl.
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Robin Gong <b38343@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/fsl-imx-cspi.txt

@@ -7,6 +7,9 @@ Required properties:
 - interrupts : Should contain CSPI/eCSPI interrupt
 - fsl,spi-num-chipselects : Contains the number of the chipselect
 - cs-gpios : Specifies the gpio pins to be used for chipselects.
+- dmas: DMA specifiers for tx and rx dma. See the DMA client binding,
+		Documentation/devicetree/bindings/dma/dma.txt
+- dma-names: DMA request names should include "tx" and "rx" if present.
 
 Example:
 
@@ -19,4 +22,6 @@ ecspi@70010000 {
 	fsl,spi-num-chipselects = <2>;
 	cs-gpios = <&gpio3 24 0>, /* GPIO3_24 */
 		   <&gpio3 25 0>; /* GPIO3_25 */
+	dmas = <&sdma 3 7 1>, <&sdma 4 7 2>;
+	dma-names = "rx", "tx";
 };