EFM32PG12_STK3402 - Correct wrong Expansion header Pin naming

Referencing to "UG257: EFM32 Pearl Gecko PG12 Starter Kit User's Guide - Chapter 4" (https://www.silabs.com/documents/public/user-guides/ug257-stk3402-usersguide.pdf) it should be like in this commit.

Author: steinerphilipp

targets/TARGET_Silicon_Labs/TARGET_EFM32/TARGET_EFM32PG12/TARGET_EFM32PG12_STK3402/PinNames.h

@@ -49,18 +49,18 @@ typedef enum {
     BUTTON2 = BTN1,
 
     /* Expansion headers */
-    EXP3  = PA8,
-    EXP4  = PC6,
-    EXP5  = PA9,
-    EXP6  = PC7,
-    EXP7  = PF3,
-    EXP8  = PC8,
-    EXP9  = PF4,
-    EXP10 = PC9,
-    EXP11 = PF5,
-    EXP12 = PA6,
-    EXP13 = PF6,
-    EXP14 = PA7,
+    EXP3  = PC9,
+    EXP4  = PA6,
+    EXP5  = PD9,
+    EXP6  = PA7,
+    EXP7  = PB6,
+    EXP8  = PA8,
+    EXP9  = PB7,
+    EXP10 = PA9,
+    EXP11 = PB8,
+    EXP12 = PD10,
+    EXP13 = PD8,
+    EXP14 = PD11,
     EXP15 = PC11,
     EXP16 = PC10,