mtd: nandsim: use NS_RAW_OFFSET()

Use NS_RAW_OFFSET() to calculate the page offset in flash RAM image by
(row, column) address.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>