mfd: support stmpe1801 18 bits enhanced port expander

Provides support for 1801 variant of stmpe gpio port expanders.
This chip has 18 gpios configurable as GPI, GPO, keypad matrix,
special key or dedicated key function.

Note that special/dedicated key function is not supported yet.

Signed-off-by: Jean-Nicolas Graux <jean-nicolas.graux@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>