OMAP4: PM: TPS6236x: speed up voltage rampup rate

Increase the voltage ramp rate to 32mv/uSec from 8mV/uSec

This should fasten up our Voltage transition times and benefit
latencies as well.

Change-Id: I5246a75c2f7ec4cc187ba8ad63a7ad71c94616da
Signed-off-by: Nishanth Menon <nm@ti.com>