ARM: dts: stm32: add gadget fifo sizes to usbotg_hs on stm32f746

USB OTG HS on STM32F746 can also be used as Peripheral (gadget), so
this patch adds DWC2 gadget mode fifo sizes bindings.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>