usb: ehci-generic: handle phy power on/off

Add generic_phy_power_on() and generic_phy_power_off()
calls to switch ON/OFF phy during probe and remove functions.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>