mtd: Flex-OneNAND support

Add support for Samsung Flex-OneNAND devices.

Flex-OneNAND combines SLC and MLC technologies into a single device.
SLC area provides increased reliability and speed, suitable for storing
code such as bootloader, kernel and root file system.  MLC area
provides high density and is suitable for storing user data.

SLC and MLC regions can be configured through kernel parameter.

[akpm@linux-foundation.org: export flexoand_region and onenand_addr]
Signed-off-by: Rohit Hagargundgi <h.rohit@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Vishak G <vishak.g@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>