mfd: Enable the STMPE MFD for Device Tree

This patch allows the STMPE Multi-Functional Device to be correctly
initialised when booting with Device Tree support enabled. Its
children are specified by the addition of subordinate devices to the
STMPE node in the Device Tree file.

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>