usb: Make compiling gadget support optional

There is no need to compile and include this code if it is not used.
CONFIG_USB_GADGET can be used for the purpose.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/configs/spear320_usbtty_defconfig b/configs/spear320_usbtty_defconfig
index 3c10f32..80acff0 100644
--- a/configs/spear320_usbtty_defconfig
+++ b/configs/spear320_usbtty_defconfig
@@ -27,3 +27,5 @@
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_MII=y
 CONFIG_CONS_INDEX=0
+CONFIG_USB=y
+CONFIG_USB_GADGET=y