boards: arm: nRF9160: avoid resizing sram0 for nRF9160_dk

sram0 node is needed to hold the size of the
total, physical SRAM available on nRF9160 SoC.
We use sram0_s to represent the Secure image
SRAM for nRF9160_dk builds.

Signed-off-by: Håkon Øye Amundsen <haakon.amundsen@nordicsemi.no>
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>