.gitignore: Add device tree vendor directories to gitignore

Vendor device tree directories shouldn't show up in the kernel git.
Add device tree vendor directories to gitignore to accomplish this.

Change-Id: I1697b38a0a018ea61eefd0d3e344dd375604388a
Signed-off-by: Venkata Narendra Kumar Gutta <vnkgutta@codeaurora.org>