[M252KG] Exclude USB UART from testing

USB UART is dedicated to USB COM and so must exclude from FPGA CI testing.

targets/TARGET_NUVOTON/TARGET_M251/pinmap.c

@@ -19,6 +19,7 @@
 #include "mbed_assert.h"
 #include "pinmap.h"
 #include "PortNames.h"
+#include "PeripheralNames.h"
 #include "mbed_error.h"
 
 /**
@@ -82,3 +83,18 @@ void pin_mode(PinName pin, PinMode mode)
      * 2. PushPullOutput/PIN_INPUT
      */
 }
+
+/* List of peripherals excluded from testing */
+const PeripheralList *pinmap_restricted_peripherals()
+{
+    static const int perifs[] = {
+        STDIO_UART          // Dedicated to USB VCOM
+    };
+
+    static const PeripheralList peripheral_list = {
+        sizeof(perifs) / sizeof(perifs[0]),
+        perifs
+    };
+
+    return &peripheral_list;
+}