MLBEDSW-2664 Clarify help for CLI-opt permanent-storage

-Make it clear that --permanent-storage option, only is valid
for Ethos-U55.
-Removed Shram from allowed values

Signed-off-by: Patrik Gustavsson <patrik.gustavsson@arm.com>
Change-Id: Ice6cacd509713e33bcb380c16dcd3c3b34a82a33
diff --git a/ethosu/vela/vela.py b/ethosu/vela/vela.py
index b1edf34..97cc873 100644
--- a/ethosu/vela/vela.py
+++ b/ethosu/vela/vela.py
@@ -183,10 +183,10 @@
         "--permanent-storage",
         default=MemArea.OffChipFlash,
         type=lambda s: MemArea[s],
-        choices=list(MemArea)[3:-1],
+        choices=list(MemArea)[3:5],
         help=(
-            "Memory area for permanent storage. To store the weights and other constant data in SRAM select "
-            "'OnChipFlash' (default: %(default)s)"
+            "Memory area for permanent storage, only valid for Ethos-U55. "
+            "To store the weights and other constant data in SRAM, select 'OnChipFlash'. (default: %(default)s)"
         ),
     )
     parser.add_argument(