usb: introduce a separate config option for DM USB device

Using CONFIG_DM_USB for this purpose prevents using DM_USB for host and not
for device.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
diff --git a/drivers/usb/Kconfig b/drivers/usb/Kconfig
index d456beb..98f8343 100644
--- a/drivers/usb/Kconfig
+++ b/drivers/usb/Kconfig
@@ -52,6 +52,20 @@
 	depends on DM_USB
 	default y
 
+config DM_USB_GADGET
+	bool "Enable driver model for USB Gadget"
+	depends on DM_USB
+	help
+	  Enable driver model for USB Gadget (Peripheral
+	  mode)
+
+config SPL_DM_USB_GADGET
+	bool "Enable driver model for USB Gadget in sPL"
+	depends on SPL_DM_USB
+	help
+	  Enable driver model for USB Gadget in SPL
+	  (Peripheral mode)
+
 source "drivers/usb/host/Kconfig"
 
 source "drivers/usb/dwc3/Kconfig"