[Controller] Added board config for custom controller STeMCell (#16287)

Co-authored-by: Mariappan Ramasamy <947300+Mariappan@users.noreply.github.com>
Co-authored-by: Mariappan Ramasamy <maari@basis-ai.com>
Co-authored-by: Sadek Baroudi <sadekbaroudi@gmail.com>

data/mappings/defaults.json

@@ -54,6 +54,12 @@
             "processor": "STM32F411",
             "bootloader": "stm32-dfu",
             "board": "BLACKPILL_STM32_F411"
+        },
+        "stemcell": {
+            "processor": "STM32F411",
+            "bootloader": "tinyuf2",
+            "board": "STEMCELL",
+            "pin_compatible": "promicro"
         }
     }
 }