mgmt: Move mcumgr into its own folder

In order to be able to add more entries under 'subsys/mgmt', move the
current contents of it, which relate exclusively to MCUMgr, to its own
folder.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/MAINTAINERS.yml b/MAINTAINERS.yml
index 4cd1278..dfbd195 100644
--- a/MAINTAINERS.yml
+++ b/MAINTAINERS.yml
@@ -841,9 +841,9 @@
     maintainers:
         - de-nordic
     files:
-        - subsys/mgmt/
-        - include/mgmt/
-        - samples/subsys/mgmt/
+        - subsys/mgmt/mcumgr/
+        - include/mgmt/mcumgr/
+        - samples/subsys/mgmt/mcumgr/
     labels:
         - "area: mcumgr"