usb: move CONFIG_USB_EHCI_MX7 to Kconfig

Create an entry for "config USB_EHCI_MX7" in Kconfig and
switch over to it for all boards.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
diff --git a/drivers/usb/host/Kconfig b/drivers/usb/host/Kconfig
index 89580cc..7992cb4 100644
--- a/drivers/usb/host/Kconfig
+++ b/drivers/usb/host/Kconfig
@@ -74,6 +74,13 @@
 	---help---
 	  Enables support for the on-chip EHCI controller on i.MX6 SoCs.
 
+config USB_EHCI_MX7
+	bool "Support for i.MX7 on-chip EHCI USB controller"
+	depends on ARCH_MX7
+	default y
+	---help---
+	  Enables support for the on-chip EHCI controller on i.MX7 SoCs.
+
 config USB_EHCI_MSM
 	bool "Support for Qualcomm on-chip EHCI USB controller"
 	depends on DM_USB