dt-bindings: mfd: Add ST Multi-Function eXpander (STMFX) core bindings

This patch adds documentation of device tree bindings for the
STMicroelectronics Multi-Function eXpander (STMFX) MFD core.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>