[MTD] [OneNAND] Add write-while-program support

OneNAND write-while-program method of writing improves performance,
compared with ordinary writes, by transferring data to OneNAND's
RAM buffers atthe same time as programming the NAND core.

When writing several NAND pages at a time, an improvement of
12% to 25% is seen.

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>