mfd: Force I2C to be built in when building WM831x

This works around issues with allmodconfig where it won't propagate the
dependency from the WM831x core back to the I2C and MFD cores.  When
doing allmodconfig this causes WM831x to be omitted and ensures that in
normal builds the dependencies get shaken out.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/Kconfig

@@ -197,7 +197,7 @@ config MFD_WM8400
 config MFD_WM831X
 	bool "Support Wolfson Microelectronics WM831x/2x PMICs"
 	select MFD_CORE
-	depends on I2C
+	depends on I2C=y
 	help
 	  Support for the Wolfson Microelecronics WM831x and WM832x PMICs.
 	  This driver provides common support for accessing the device,