boards: seagate: legend: Add board revision configurations

board.yml file was missing the list of custom revisions of this board.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
(cherry picked from commit 9c803c02b5)

boards/seagate/legend/board.yml

@@ -4,5 +4,10 @@ board:
   vendor: seagate
   revision:
     format: custom
+    default: "25hdd"
+    revisions:
+      - name: "25hdd"
+      - name: "25ssd"
+      - name: "35"
   socs:
     - name: stm32f070xb