mtd: nand: atmel_nand: retrieve NFC clock

Retrieve the NFC clock to make sure it is enabled. Make that optional to ensure
compatibility with previous device trees but document it as mandatory so newer
device trees will include it.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>