[media] MAINTAINERS: Add the OMAP4 ISS driver

Update the OMAP Image Signal Processor entry to cover both the OMAP3 ISP
and OMAP4 ISS.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>