mtd: Add DiskOnChip G3 support

Add support for DiskOnChip G3 chips. The support is quite
limited yet :
 - no flash writes/erases are implemented
 - ECC fixes are not implemented
 - powerdown is not implemented
 - IPL handling is not yet done

On the brighter side, the chip reading does work.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>