MAINTAINERS: Update AT91 entry

Jean Christophe has not been active on the mailing lists for a while.
Remove him from the maintainers

Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>