boards: nrf9160: update secure image SRAM size to work with TF-M

Update the default Secure SRAM partition size for nRF9160,
so it aligns with the default TF-M secure partition size.
Also, change the DeviceTree node name from "bsd" to "modem",
to align with how the respective library component is named.

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