Documentation: Describe bindings for STMPE Multi-Functional Device driver

Here we add the required documentation for the STMPE Multi-Functional
Device (MFD) Device Tree bindings. It describes all of the bindings
currently supported by the driver.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>