dt-bindings: ARM: at91: update maintainers entry

Align the binding documentation with the newly updated MAINTAINERS
entry.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/5bf9873eeee3cd49c52a8952a7cd4cb60b61d50a.1643553501.git.nicolas.ferre@microchip.com