mfd: simple-mfd-i2c: Select MFD_CORE to fix build error

MFD_SIMPLE_MFD_I2C should select the MFD_CORE to a prevent build error:

aarch64-linux-ld: drivers/mfd/simple-mfd-i2c.o: in function `simple_mfd_i2c_probe':
drivers/mfd/simple-mfd-i2c.c:55: undefined reference to `devm_mfd_add_devices'

Cc: <[e-mail]>
Fixes: c753ea31781aa ("mfd: simple-mfd-i2c: Add support for registering devices via MFD cells")
Signed-off-by: Robert Marko <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/mfd/Kconfig

···
 config MFD_SIMPLE_MFD_I2C
 	tristate
 	depends on I2C
+	select MFD_CORE
 	select REGMAP_I2C
 	help
 	  This driver creates a single register map with the intention for it