ARM: OMAP3: VC: calculate ramp times

OMAP3 VC code now uses voltage deltas + slew rates for calculating actual
ramp times for voltage changes. Previously a static value was used.
Two calculation methods are provided: i2c_timings and off_timings.
I2C timings are used during retention or off mode transition which
is initiated over I2C, and OFF timings are used if PMIC signal
(nsleep) is used to control all the off mode voltages at the same time.

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