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/kp_imx53_defconfig b/configs/kp_imx53_defconfig
index 45227c2..225be66 100644
--- a/configs/kp_imx53_defconfig
+++ b/configs/kp_imx53_defconfig
@@ -45,4 +45,5 @@
 CONFIG_CONS_INDEX=2
 CONFIG_MXC_UART=y
 CONFIG_USB=y
+CONFIG_USB_EHCI_MX5=y
 CONFIG_USB_STORAGE=y