ARM: dts: msm: Add support for msm8926 v2

Distinguish between msm8926 v1 and v2 based boards.
Add support for boards with external VDD_APC buck regulator.

CRs-fixed: 605364
Change-Id: I4287a0a55547f16c8b3977ca8745f053cb03b76a
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>