gpio: stmpe: enforce device tree only mode

Require that device tree be used with STMPE (all platforms use this)
and enforce OF_GPIO, then delete the platform data.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

diff --cc drivers/gpio/Kconfig
Simple merge

diff --cc drivers/gpio/gpio-stmpe.c
Simple merge

diff --cc include/linux/mfd/stmpe.h
Simple merge