mfd: stmpe: Probe properly from the Device Tree

The current STMPE I2C probing code does not really match the
compatible strings - it matches node names happening to give
the right device name. Instead, let's introduce some real
compatible matching, more complex, more accurate. Make the
driver depend on OF since all platforms using it are DT-only.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>