.gitignore: Ignore vendor devicetree directories from git

As the vendor devicetree directories are softlinked, ignore them through
git. Remove "/" in the end for arm64 for properly ignoring.

Change-Id: Ib6c9c31ecd2322f93609f72cbfb0143bd042a649
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>