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.

Change-Id: Ic70e933daf6b6f3bafc0f0fe77064c8395e611d1
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>