ARM: dts: stm32: add joystick support on stm32746g-eval

The joystick (B3) on stm32746g-eval uses gpios on STMFX gpio expander.
These gpios need a pin configuration (push-pull and bias-pull-up),
described under stmfx_pinctrl node.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>