Documentation: dt: Add stmpe1600 compatible string to STMPE MFD

This patch adds a new compatible string for stmpe mfd to support
stmpe1600 variant.

Signed-off-by: Amelie DELAUNAY <amelie.delaunay@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>