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 Behún <marek.behun@nic.cz>