atmel: fix xplained documentation

Documentation is referring to nandflash.txt which doesn't exist.
Instead, readme.txt provides the instructions to flash the NAND storage.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>