mtd: atmel_nand: optimize read/write buffer functions

For PIO NAND access functions, we use the features of the SMC:
- no need to take into account the NAND bus width: SMC will deal with this
- use of an IO memcpy on the NAND chip-select space is able to generate
  proper SMC behavior.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>