ANDROID: GKI: power: supply: Add VOLTAGE_STEP property

This property exposes the step size (in uV) to be used while
incrementing or decrementing voltage of a PSY.

Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
(cherry picked from commit 4cde627bd0b8a987bf4104ce07a33ee87c390a44)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 150789066
Change-Id: Ic70e933daf6b6f3bafc0f0fe77064c8395e611d1