power: qpnp-qg: Add a DT property to use S7 PON OCV

PMI632 PON OCV is dependent on the primary PMIC it is
paired with. Add a DT property to force using S7 based
OCV instead of the multi-level PON OCV logic.

Change-Id: I3846276646e933c4c893b7325bf70ea2455dc891
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>