checkpatch: check vendor compatible with dashes

The current vendor compatible check will not match vendors with dashes,
like:

  compatible="asahi-kasei"

Change-Id: Ib0773ea8f12b38a1ffc6fe7dd7ce900e8c3aa304
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Reported-by: Joe Perches <joe@perches.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: 8f0dbfaf2715b80f491c3e23c318c4202abf89e2
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>