ARM: msm: Create a common board-dt and config ARCH_MSM_DT

Create a config ARCH_MSM_DT and common board-dt.c
to encapsulate the MSM DT based targets.

Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>