mfd: Modify samsung mfd driver for common api

Previous naming rule of samsung pmic start with s5m prefix.
But It is changed by s2m.
To cover various samsung s2m and s5m series,
This patch modify function and variable name for common usage.

Signed-off-by: Sangbeom Kim <sbkim73@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>