MAINTAINERS: ARM: at91: add co-maintainer for ARM/Microchip

Add Ludovic as a new co-maintainer for the AT91 Microchip ARM
sub-architecture.
Add the newly created kernel.org group git tree that we will use
from now on.

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