mfd: Don't ask about the TPS65912 core driver in Kconfig

The user has to select the I2C and SPI drivers individually and they select
the core driver for the device so there's no point in presenting the user
with an option for the core driver.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/Kconfig

@@ -172,7 +172,7 @@ config MFD_TPS6586X
 	  will be called tps6586x.
 
 config MFD_TPS65912
-	bool "TPS65912 PMIC"
+	bool
 	depends on GPIOLIB
 
 config MFD_TPS65912_I2C