stm32mp: cmd_stm32prog: add dependencies with USB_GADGET_DOWNLOAD

This patch avoids compilation issue when CONFIG_USB_GADGET is deactivated
in defconfig, with undefined reference to run_usb_dnl_gadget and to
g_dnl_set_product.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/arch/arm/mach-stm32mp/cmd_stm32prog/Kconfig b/arch/arm/mach-stm32mp/cmd_stm32prog/Kconfig
index 8f91db4..5892762 100644
--- a/arch/arm/mach-stm32mp/cmd_stm32prog/Kconfig
+++ b/arch/arm/mach-stm32mp/cmd_stm32prog/Kconfig
@@ -17,6 +17,7 @@
 config CMD_STM32PROG_USB
 	bool "support stm32prog over USB"
 	depends on CMD_STM32PROG
+	depends on USB_GADGET_DOWNLOAD
 	default y
 	help
 		activate the command "stm32prog usb" for STM32MP soc family