mfd: Add STMPE1600 support

STMPE1600 is a 16-bit port expander.
Datasheet is available here :
http://www2.st.com/content/st_com/en/products/interfaces-and-transceivers/


i-o-expanders-and-level-translators/i-o-expanders/stmpe1600.html

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