mtd: atmel_nand: use CPU I/O when buffer is in vmalloc(ed) region

The previous way of dealing with vmalloc(ed) region by walking
though the pages can not work well actually. We just fall back
to CPU I/O when the buffer address is higher than `high_memory'.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Hong Xu <hong.xu@atmel.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>