Kconfig: gadget: Move CONFIG_USB_FUNCTION_THOR to Kconfig

This commit moves USB_FUNCTION_THOR config to Kconfig.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/configs/zynq_z_turn_defconfig b/configs/zynq_z_turn_defconfig
index c315b91..68a2208 100644
--- a/configs/zynq_z_turn_defconfig
+++ b/configs/zynq_z_turn_defconfig
@@ -58,3 +58,4 @@
 CONFIG_USB_GADGET_PRODUCT_NUM=0x0300
 CONFIG_CI_UDC=y
 CONFIG_USB_GADGET_DOWNLOAD=y
+CONFIG_USB_FUNCTION_THOR=y