mmc: davinci_mmc: add DT support

Adds device tree support for davinci_mmc. Also add binding documentation.
As of now tested for non-dma PIO mode and without GPIO card_detect/
write_protect option because of dependencies on EDMA and GPIO module DT
support.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Sekhar Nori <nsekhar@ti.com>
Cc: linux-mmc@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: davinci-linux-open-source@linux.davincidsp.com
Cc: devicetree-discuss@lists.ozlabs.org
Cc: cjb@laptop.org
Cc: Sekhar Nori <nsekhar@ti.com>
Cc: mporter@ti.com
Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>