MAINTAINERS: at91: change email address

Following the Microchip / Atmel merger and the unification of internal IT, it's
more convenient for me to swith to the microchip.com address. Change all my
entries to reflect this.

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