da850: Add instructions to copy AIS image to an MMC card

The da850 soc's can boot from a external mmc card, but
the AIS image should be written to the correct sector.

Add instructions to copy the AIS image to a MMC card.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>