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/arch/arm/Kconfig b/arch/arm/Kconfig
index 13ba774..7f513ce 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -932,6 +932,7 @@
 	select DM_SERIAL
 	select OF_CONTROL
 	imply CMD_DM
+	imply DM_USB_GADGET
 
 config ARCH_ZYNQMP
 	bool "Xilinx ZynqMP based platform"
@@ -949,6 +950,7 @@
 	imply CMD_DM
 	imply FAT_WRITE
 	imply MP
+	imply DM_USB_GADGET
 
 config TEGRA
 	bool "NVIDIA Tegra"