platform: Add weak symbol for hs only mode

Some platforms use usb 3.0 but do not have qmp phy support. This
leads to undefined reference linker errors when compiled in usb3.0
driver. Add weak symbol to resolve the link errors.

Change-Id: I591eb1bd82c09627198c6d6b5060c9497d0afadc
diff --git a/platform/init.c b/platform/init.c
index aeb4f20..34c5726 100644
--- a/platform/init.c
+++ b/platform/init.c
@@ -1,7 +1,7 @@
 /*
  * Copyright (c) 2008 Travis Geiselbrecht
  *
- * Copyright (c) 2014, The Linux Foundation. All rights reserved.
+ * Copyright (c) 2014-2015, The Linux Foundation. All rights reserved.
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files
@@ -140,3 +140,8 @@
 {
 	return 0;
 }
+
+__WEAK uint32_t use_hsonly_mode()
+{
+	return 0;
+}