MAINTAINERS: replace an AT91 maintainer

As some help is needed from an active maintainer, replace Andrew Victor
by Alexandre Belloni in the ARM/Atmel MAINTAINERS' entry (aka AT91).
Add an entry to the CREDITS file.

Thanks Andrew for the great role you played during the early days of this
product family.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Andrew Victor <linux@maxim.org.za>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>