spi: davinci: let DMA operation be specified on per-device basis

Let DMA operation be specified on a per-device basis instead
of selecting it once during probe.

A side effect of this is the need to combine the PIO and DMA buffer
txrx_bufs routine. This is good since they anyway share some common
functionality.
Signed-off-by: Brian Niebuhr <bniebuhr@efjohnson.com>
Tested-By: Michael Williamson <michael.williamson@criticallink.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>

arch/arm/mach-davinci/include/mach/spi.h

@@ -41,6 +41,7 @@ struct davinci_spi_config {
 	u8	parity_enable;
 #define SPI_IO_TYPE_INTR	0
 #define SPI_IO_TYPE_POLL	1
+#define SPI_IO_TYPE_DMA		2
 	u8	io_type;
 	u8	timer_disable;
 	u8	c2tdelay;