configs/atmel_sama5d3xek: bump to linux4sam_5.5

Bump to linux4sam_5.5 and remove dtb variants which are no longer
supported. This is needed to fix the build with gcc 5.x.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>