MAINTAINERS: update entry for Microchip NAND driver support

Replace the Microchip/Atmel NAND controller driver maintainer
by removing Josh and adding Tudor.

Cc: Josh Wu <rainyfeeling@outlook.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>