system_off fails to go into soft_off (deep sleep) state on cc1352r1_launchxl

[vanti]

Describe the bug
On cc1352r1_launchxl, the system fails to enter soft_off state (what used to be known as deep sleep 1):

*** Booting Zephyr OS build v2.5.0-rc1-106-g2712d74cdcbf  ***

cc1352r1_launchxl system off demo
Busy-wait 5 s
Sleep 2000 us (IDLE)
Sleep 3 s (STANDBY)
Entering system off (SHUTDOWN); press BUTTON1 to restart
ERROR: System off failed

To Reproduce
Steps to reproduce the behavior:

1. cd samples/boards/ti/cc13x2_cc26x2/system_off
2. west build -b cc1352r1_launchxl
3. west flash
4. See error

Expected behavior
The system should enter soft_off state.

Impact
Cannot enter soft_off power state to save power when desired.

Environment (please complete the following information):

• OS: (e.g. Linux, MacOS, Windows): Ubuntu 18.04
• Toolchain (e.g Zephyr SDK, ...): Zephyr SDK 0.11.4
• Commit SHA or Version used: 2712d74