dts: update memory map and remove ext-uicr

Extended UICR will not be used as its configurations will be merged
with the UICR registers in NVR.

Memory maps changes are needed to align with pre compiled
firmware.

Signed-off-by: Håkon Amundsen <haakon.amundsen@nordicsemi.no>
Signed-off-by: Jonathan Nilsen <jonathan.nilsen@nordicsemi.no>