Fixes the following randconfig build error:
 ld: drivers/mfd/simple-mfd-i2c.o: in function `simple_mfd_i2c_probe':
 simple-mfd-i2c.c:(.text+0x48): undefined reference to `__devm_regmap_init_i2c'
 ld: drivers/mfd/simple-mfd-i2c.o: in function `simple_mfd_i2c_driver_init':
 simple-mfd-i2c.c:(.init.text+0x14): undefined reference to `i2c_register_driver'
 ld: drivers/mfd/simple-mfd-i2c.o: in function `simple_mfd_i2c_driver_exit':
 simple-mfd-i2c.c:(.exit.text+0xd): undefined reference to `i2c_del_driver'
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
 
 config MFD_SL28CPLD
        tristate "Kontron sl28cpld Board Management Controller"
+       depends on I2C
        select MFD_SIMPLE_MFD_I2C
        help
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