Add define for PN1803/L508/L509

Change-Id: Ide7bf05b34d2e4b6c9a7259032593e40d366c8eb
diff --git a/mbtk/mbtk_lib/src/mbtk_gpio.c b/mbtk/mbtk_lib/src/mbtk_gpio.c
index ed48796..26c9f24 100755
--- a/mbtk/mbtk_lib/src/mbtk_gpio.c
+++ b/mbtk/mbtk_lib/src/mbtk_gpio.c
@@ -513,6 +513,7 @@
     return ret;
 }
 
+#if defined(MBTK_PROJECT_PN1803)
 int mbtk_at_gpio(void* arg)
 {
     /*
@@ -570,3 +571,26 @@
     mbtk_adc_close();
     return n;
 }
+
+#elif defined(MBTK_PROJECT_L508)
+
+int mbtk_at_gpio(void* arg)
+{
+    return -1;
+}
+
+#elif defined(MBTK_PROJECT_L509)
+
+int mbtk_at_gpio(void* arg)
+{
+    return -1;
+}
+
+#else
+
+int mbtk_at_gpio(void* arg)
+{
+    return -1;
+}
+
+#endif