Revert "OMAP3+: OPP: Remove unused _volt_to_opp() function"

This reverts commit 3d03e7f2bcd60a01de419d10daea17b53f3a1b13.
Revert "OMAP3+: DVFS: Use opp_find_volt functions instead of _volt_to_opp"

This reverts commit c6289633fa11288f800b317d4140695db6c4c7f0.
Revert "OMAP3+: voltage: omap_voltage_get_voltdata searches only for Vnom"

This reverts commit 8cde30a73b68f4e2855c752689c4448b65cc7064.