arm/dt: msm8226: refactor device tree into SOC and board specific files

With the upcoming 8x26 variants, several targets will use the same device
tree files with specific overrides. Break up the device tree files into a
SOC include file and a board include file. Each new target can then select
its own include files and provided board specific overrides in the top
level DTS file.

Change-Id: Ib076f862618df89c1a609376068733f4a00dabad
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>