The MAX8998 chip have regulator and rtc features. The i2c slave address
of regulator and rtc is different, so needs each i2c client on i2c
operation functions.

Also, this patch exports i2c operation functions instead of callback to
make easy to read.
Signed-off-by: NJoonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>