meta-phosphor: repotest: prohibit new DTS files

DTS files should be submitted upstream to the kernel, just like patches.
Allowing individual DTS files to be added to the tree is just another
form of patching the kernel.  Explicitly search and prohibit new ones
from being added.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ibd176423cda5082ead3bf1d3da1e66507e0ea475