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: 52b461b86a9f ("mfd: Add regmap cache support for wm8350")
Signed-off-by: Uwe Kleine-König <[e-mail]>
Acked-by: Charles Keepax <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/Kconfig

···
 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