boards: arm: nrf9160_dk: clean up sram0_ns and sram0_bsd definitions

We do not want sram0_ns and sram0_bsd to represent physical
ram; these are just portions of sram reserved for the non-secure
image and the bsd library, respectively. Thus we can remove the
compatible property from these nodes. We also make use of
'reserved-memory' to represent the different memory partitions
to be used by the nrf9160 builds.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>