dts: arm: st: add property maximum-speed

On STM32 USB, there are currently four types
of maximum-speed: "super-speed", "high-speed",
"full-speed" and "low-speed" (if present).
To simplify the driver, add maximum-speed if
not present as a dts property.

Signed-off-by: Khaoula Bidani <khaoula.bidani-ext@st.com>