boards: nrf9160: fix default flash partitioning

This commit fixes the default partitioning of flash
in nrf9160 Cortex-M33 MCU, to comply with the nRF SPU
region boundaries. To properly align the partition start
addresses with SPU requirements we modify the start of the
slot0 and slot1 partitions. This leaves some extra space
available for MCUboot, whose size is now changed from
0xc000 to 0x10000. In addition, the non-secure partition
size (slot0_nonsecure) is decreased, and as a consequence
of that we also modify the board's .yaml file to reflect
the new image size of the non-secure version of the board.

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