Add Synology Rs2421rp+ (#480)

* Added new APC unit SMT1500RM2UC

device-types/Synology/RS2421RP+.yaml

@@ -0,0 +1,22 @@
+---
+manufacturer: Synology
+model: RS2421RP+
+slug: synology-rs2421rp-plus
+part_number: RS2421RP+
+u_height: 2
+is_full_depth: true
+comments: '[RS2421RP Series Datasheet](https://www.synology.com/en-us/products/RS2421+#specs)'
+power-ports:
+  - name: PSU 1
+    type: iec-60320-c14
+  - name: PSU 2
+    type: iec-60320-c14
+interfaces:
+  - name: Ethernet 1
+    type: 1000base-t
+  - name: Ethernet 2
+    type: 1000base-t
+  - name: Ethernet 3
+    type: 1000base-t
+  - name: Ethernet 4
+    type: 1000base-t