ARM: OMAP4: VC: calculate ramp times

OMAP4 VC code now uses voltage deltas + slew rates for calculating
actual ramp times for voltage changes. Both retention / sleep +
off mode voltage ramp times are setup at the same time during
initialization.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>