Rename lantiq dts files with soc name being the prefix, so that the
board dts file can be located easily by soc name, and we also gain
the consistency of naming.

This patch is a preparation for the addition of dts for newer SoCs
(Lantiq ARX100, VRX200 and xRX330).
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>