OMAP4: PM: TWL6030: fix voltage conversion formula

omap_twl_vsel_to_uv() and omap_twl_uv_to_vsel() functions used to convert
voltages to TWL6030 SMPS commands (a.k.a "vsel") implement incorrect conversion
formula.
It uses legacy OMAP3 formula, but OMAP4 Power IC has different offset and
voltage step:
 - Voltage Step is now 12.66mV (instead of 12.5mV)
 - Offset is either 607.7mV or 709mV depending on TWL6030 chip revision
   (instead of 600mV)
This leads to setting voltages potentially higher than expected, and so
potentially some (limited) power overconsumption.

For reference, see formula and tables in section 8.5.2.3
"Output Voltage Selection (Standard Mode / Extended Mode with or without offset)"
 in TWL6030 functional specifications document.

[nm@ti.com: ported to voltdm_c]
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Patrick Titiano <p-titiano@ti.com>