MAINTAINERS: net: macb: add Claudiu as co-maintainer

I would like that Claudiu becomes co-maintainer of the Cadence macb
driver. He's already participating to lots of reviews and enhancements
to this driver and knows the different versions of this controller.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>