spi: omap2-mcspi: add DMA engine support

Add DMA engine support to the OMAP SPI driver.  This supplements the
private DMA API implementation contained within this driver, and the
driver can be independently switched at build time between using DMA
engine and the private DMA API for the transmit and receive sides.

Tested-by: Shubhrajyoti <shubhrajyoti@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>