Convert CONFIG_USB_EHCI_MX5 to Kconfig

This converts the following to Kconfig:
   CONFIG_USB_EHCI_MX5

Signed-off-by: Lukasz Majewski <lukma@denx.de>
diff --git a/configs/mx51evk_defconfig b/configs/mx51evk_defconfig
index dc0b3b3..78aa89d 100644
--- a/configs/mx51evk_defconfig
+++ b/configs/mx51evk_defconfig
@@ -26,6 +26,7 @@
 CONFIG_SPI=y
 CONFIG_MXC_SPI=y
 CONFIG_USB=y
+CONFIG_USB_EHCI_MX5=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_HOST_ETHER=y
 CONFIG_USB_ETHER_ASIX=y