MAINTAINERS: Atmel drivers: change NAND and ISI entries

Update Josh's entries about NAND and ISI drivers.
Thanks for your work with Atmel Josh!

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Josh Wu <rainyfeeling@outlook.com>