target: weak function to select the default usb controller

Introduced a weak function to select the chip idea usb controller
at runtime. Target code can override this to select other
controllers to be used for specific targets.

Change-Id: I6be195b590f972c863863d96b13e75e3db26737e
diff --git a/target/init.c b/target/init.c
index a4188a8..9308861 100644
--- a/target/init.c
+++ b/target/init.c
@@ -155,3 +155,9 @@
 {
 	return 0;
 }
+
+/* default usb controller to be used. */
+__WEAK const char * target_usb_controller()
+{
+	return "ci";
+}