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/spear600_usbtty_nand_defconfig b/configs/spear600_usbtty_nand_defconfig
index 078e934..ccbfec9 100644
--- a/configs/spear600_usbtty_nand_defconfig
+++ b/configs/spear600_usbtty_nand_defconfig
@@ -28,3 +28,5 @@
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_MII=y
 CONFIG_CONS_INDEX=0
+CONFIG_USB=y
+CONFIG_USB_GADGET=y