regulator: core: Change the unit of ramp_delay from mV/uS to uV/uS

This change makes it possible to set ramp_delay with 0.xxx mV/uS without
truncation issue.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>