usb: dwc3: Move CONFIG_USB_DWC3_GADGET/HOST to Kconfig

Description was borrowed from kernel dwc3 Kconfig.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
diff --git a/include/configs/am43xx_evm.h b/include/configs/am43xx_evm.h
index 3643489..5e0f65d 100644
--- a/include/configs/am43xx_evm.h
+++ b/include/configs/am43xx_evm.h
@@ -124,6 +124,7 @@
 /* USB GADGET */
 #if defined(CONFIG_SPL_BUILD) && !defined(CONFIG_SPL_USBETH_SUPPORT)
 #undef CONFIG_USB_DWC3
+#undef CONFIG_USB_DWC3_GADGET
 
 #undef CONFIG_USB_GADGET_DOWNLOAD
 #undef CONFIG_USB_GADGET_VBUS_DRAW
@@ -134,7 +135,6 @@
 	(defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_USBETH_SUPPORT))
 #define CONFIG_USB_DWC3_PHY_OMAP
 #define CONFIG_USB_DWC3_OMAP
-#define CONFIG_USB_DWC3_GADGET
 
 #define CONFIG_G_DNL_MANUFACTURER "Texas Instruments"
 #define CONFIG_G_DNL_VENDOR_NUM 0x0403