power: batterydata-lib: Add an API to interpolate OCV-SOC slope

Slope of a SOC-OCV curve is mathematically defined as
(OCV1 - OCV2) / (SOC1 - SOC2). It defines the rate of
change of OCV across SOC.

Change-Id: Iad1aed4fbccb45488109aae8713b1fc9accebe90
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>