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: <stable@vger.kernel.org>
Fixes: c753ea31 ("mfd: simple-mfd-i2c: Add support for registering devices via MFD cells")
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211102100420.112215-1-robert.marko@sartura.hr