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

This patch adds support for STMicroelectronics Multi-Function eXpander
(STMFX) on stm32746g-eval. It is connected on i2c1.

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>