west.yml: update to current Zephyr

Update to Zephyr commit 641318561993 ("drivers: flash: nordic: fix
nrf-mramc indentation").

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

Author: lyakh

west.yml

@@ -43,7 +43,7 @@ manifest:
 
     - name: zephyr
       repo-path: zephyr
-      revision: 9075d5335596080291ebcc448819fed2110dcb9a
+      revision: 64131856199342d0cb6507e5c57355c82f24dd69
       remote: zephyrproject
 
       # Import some projects listed in zephyr/west.yml@revision