mtd: nand/fsmc: Access the NAND device word by word whenever possible
The default way of accessing nand device is using the nand width. This means
that 8bit devices are using u8 * and 16bit devices are accessed using u16 *.
This results in a non-optimal performance since the FSMC is designed to
translate the normal word accesses into device width based accesses. This patch
implements read_buf and write_buf callbacks using word by word accesses.