mmc: omap_hsmmc: add DMA engine support

Add DMA engine support to the OMAP HSMMC driver.  This supplements the
private DMA API implementation contained within this driver, and the
driver can be switched at build time between using DMA engine and the
private DMA API.

Tested-by: Grazvydas Ignotas <notasas@gmail.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>