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/configs/am43xx_evm_defconfig b/configs/am43xx_evm_defconfig
index 942a6a0..5406beb 100644
--- a/configs/am43xx_evm_defconfig
+++ b/configs/am43xx_evm_defconfig
@@ -13,6 +13,7 @@
 CONFIG_TI_QSPI=y
 CONFIG_USB=y
 CONFIG_USB_DWC3=y
+CONFIG_USB_DWC3_GADGET=y
 CONFIG_USB_GADGET=y
 CONFIG_USB_GADGET_DOWNLOAD=y
 CONFIG_OF_LIBFDT=y