regulator: fan53555: update slew rate according to voltage step

Currently, the voltage transition slew rate setting for fan53555 is
based on 10 mV voltage scaling step. For regulator with a scaling step
other than 10 mV, the slew rate has a relationship between voltage
scaling step and the slew rate based on 10 mV voltage scaling step.

Change the slew rate setting according to the voltage scaling step, and
round it to a lower avaiable value if it is an itermediate one.

Change-Id: Iccb13ebcb4bac15d76420602363bdcdfe5fc0a2d
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>