MAINTAINERS: update Microchip AT91 sound entries with documentation files

Add documentation files to Microchip AT91 sound entries.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20230301113807.24036-9-claudiu.beznea@microchip.com


Signed-off-by: Mark Brown <broonie@kernel.org>