configs/atmel: bump to linux4sam_6.1

configs/atmel: bump to linux4sam_6.1

Only the atmel_sama5d27_som1 board was bumped to linux4sam_6.1 so update
the other boards.

Moreover, display variant device trees are no longer in the linux4sam
kernel tree as we have adopted the DT overlays. In case of interest for those
DTs, please use the Microchip external available here:
https://github.com/linux4sam/buildroot-external-microchip

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>