ARM: mxs_defconfig: add CONFIG_USB_PHY

Commit edc7cb2e (usb: phy: make it a menuconfig) makes USB_MXS_PHY
be a sub-item of menuconfig symbol USB_PHY.  This change gets the
selection of CONFIG_USB_MXS_PHY in mxs_defconfig lost.  Hence the
boot stops at the point below.

  [    1.600867] ci_hdrc ci_hdrc.0: doesn't support gadget
  [    1.606282] ci_hdrc ci_hdrc.0: EHCI Host Controller
  [    1.613522] ci_hdrc ci_hdrc.0: new USB bus registered, assigned bus number 1

Add CONFIG_USB_PHY to have the CONFIG_USB_MXS_PHY selection back to
work.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>