mfd: Rename s5m file and directories to samsung

Previously, Samsung PMIC naming rule start with prefix of s5m.
But Naming rule is changed.
From now on, Prefix will be changed to s2m.
So, To support pmic series of s5m and s2m, change mfd file and directory name.

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