mfd: wm8350-i2c: Make sure the i2c regmap functions are compiled

This fixes a compile failure:

	drivers/built-in.o: In function `wm8350_i2c_probe':
	core.c:(.text+0x828b0): undefined reference to `__devm_regmap_init_i2c'
	Makefile:953: recipe for target 'vmlinux' failed

Fixes: 52b461b8 ("mfd: Add regmap cache support for wm8350")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/Kconfig

@@ -1573,6 +1573,7 @@ config MFD_WM8350
 config MFD_WM8350_I2C
 	bool "Wolfson Microelectronics WM8350 with I2C"
 	select MFD_WM8350
+	select REGMAP_I2C
 	depends on I2C=y
 	help
 	  The WM8350 is an integrated audio and power management