manifest: Update sdk-zephyr with watchdog tests for error cases

Include changes from:
nrfconnect/sdk-zephyr#1640

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>

west.yml

@@ -63,7 +63,7 @@ manifest:
     # https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/zephyr/guides/modules.html
     - name: zephyr
       repo-path: sdk-zephyr
-      revision: b5cf3040298410845311ff7397374540d3451528
+      revision: pull/1640/head
       import:
         # In addition to the zephyr repository itself, NCS also
         # imports the contents of zephyr/west.yml at the above