[media] MAINTAINERS: atmel-isc: add entry for Atmel ISC

Add the MAINTAINERS' entry for Microchip / Atmel Image Sensor Controller.

Signed-off-by: Songjun Wu <songjun.wu@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>