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 --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index 4187fcb..22b4656 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -644,6 +644,7 @@
 config GPIO_STMPE
 	bool "STMPE GPIOs"
 	depends on MFD_STMPE
+	depends on OF_GPIO
 	select GPIOLIB_IRQCHIP
 	help
 	  This enables support for the GPIOs found on the STMPE I/O