MAINTAINERS: mtd/nand: update Microchip nand entry

Update Wenyou Yang email address.
Take advantage of this update to move this entry to the MICROCHIP / ATMEL
location and add the DT binding documentation link.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Wenyou Yang <wenyou.yang@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>