power: qpnp-qg: Add support for voltage-only mode of QG

Add support for voltage-only mode for QG. The mode is enabled
by a DT property "qcom,qg-vbms-mode".

CRs-Fixed: 2374455
Change-Id: Ic4af12929d68d8a97872535feed72480e628a536
Signed-off-by: Vamshi Krishna B V <vbv@codeaurora.org>