boards: nordic: nrf54l05: Change to use common dts partitioning file

Adds a common vendor dts file specifying the default partition
layout for nRF54L05-based cpuapp board targets and updates boards
to use this common file. This also drops the secure/non-secure split
in the partitioning as this was reducing NVM storage that is not
used by this board target and fixes the wrong flash field in
twister yaml files

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>