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>
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index 3383412..29be025 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -675,6 +675,7 @@
 config MFD_STMPE
 	bool "STMicroelectronics STMPE"
 	depends on (I2C=y || SPI_MASTER=y)
+	depends on OF
 	select MFD_CORE
 	help
 	  Support for the STMPE family of I/O Expanders from