MAINTAINERS: remove the / ATMEL string from MICROCHIP entries

No need to keep this additional string as it can puzzle people
while adding new driver's entries.
Move the NAND entry to keep it alphabetically ordered.

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