ARM: multi_v7_defconfig: Enable stih415/6 usb2 phy driver.

This driver is used by the ehci / ohci usb controllers on stih415/6 SoCs.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>