ARM: msm: dts: Remove SDM640 Device Tree Overlay Support

When compiling for SDM640, with device tree overlay support
enabled, compilation will fail due to the absence of
required overlay and device tree source files. Remove
device tree overlay support for target SDM640,
as it is not appropriate to have device tree overlay support
for it if the required files are missing.

Change-Id: Ifff3b9fd2f0016f92eeb54cc5fb8d07b1d45a740
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>