boards: arm: nrf: add non-secure SRAM memory information in DTS

When we build Zephyr as Secure image on nRF340 Application
MCU and nRF9160 SoC we would like to pass the information
about the reserved memory area allocated to the Non-Secure
images. The information may be needed to apply proper
security configuration. We add a "chosen" node in board .dts
file for this purpose.

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