OMAP3+: voltage: rename scale and reset functions using voltdm_ prefix

Rename voltage scaling related functions to use voltdm_ prefix intead
of omap_voltage_, and cleanup kerneldoc comments in the process.

s/omap_voltage_scale_vdd/voltdm_scale/
s/omap_voltage_reset/voltdm_reset/

Also, in voltdm_reset() s/target_uvdc/target_volt/ to be consistent with
naming throughout the file.

No functional changes.

Signed-off-by: Kevin Hilman <khilman@ti.com>