dts/boards: nordic: nrf5340: Split partition into s and ns versions

Splits up partition configuration for nrf5340-based cpuapp board
targets for secure and non-secure versions, the secure version now
has an extra 16KiB per slot which was previously wrongly reserved
for TF-M partitions which the secure board target does not use

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