msm: 8929: Add a dedicated DT_MACHINE for msm8929

Create an 8929 specific machine macro to support
dynamic selection of required DT_MACHINE at runtime
according to device tree.

Change-Id: I1105c534b95a20cc276c3bded244259d59dcefa7
Signed-off-by: Kishan Kumar <kishank@codeaurora.org>