Convert CONFIG_USB_EHCI_MXS to Kconfig

This option is only used for
  mx23evk_defconfig
  mx23_olinuxino_defconfig
which are the only i.MX23 boards.

Add depend on ARCH_MX23 and default to y.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
diff --git a/drivers/usb/host/Kconfig b/drivers/usb/host/Kconfig
index 10b0479..be5e4b9 100644
--- a/drivers/usb/host/Kconfig
+++ b/drivers/usb/host/Kconfig
@@ -175,6 +175,13 @@
 	---help---
 	  Enables support for the on-chip EHCI controller on i.MX7 SoCs.
 
+config USB_EHCI_MXS
+	bool "Support for i.MX23 EHCI USB controller"
+	depends on ARCH_MX23
+	default y
+	help
+	  Enables support for the on-chip EHCI controller on i.MX23 SoCs.
+
 config USB_EHCI_OMAP
 	bool "Support for OMAP3+ on-chip EHCI USB controller"
 	depends on ARCH_OMAP2PLUS