msm: 8974: Create a common board-dt and cleanup 8974

MSM DT targets will use a per SoC machine macro.
Create a 8974 specific machine macro and create a
common board-dt.c file.
Make the naming consistent for 8974.

Change-Id: I8ecfe840f614e7a60cf8430088c43c8e08be75ec
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>