ARM: dts: msm: move external power supply configuration for msm8926-v2

Move external VDD_APC power supply configurations from msm8926 to
msm8926-v2-ext-buck file as the external power supply only exists on some
msm8926-v2 boards.

CRs-fixed: 607899
Change-Id: I73d333b8b7d0489f06fa1f5e34a8cb9576a86062
Signed-off-by: Ke Liu <keliu@codeaurora.org>