This patch fixes DT vendor-prefixes.txt documentation to reference
isil (NASDAQ symbol and the most used prefix inside the kernel) for
Intersil.

It reverts 7c75c1d5 ("dt-bindings: Document deprecated device
vendor name to fix related warning").
Signed-off-by: NArnaud Ebalard <arno@natisbad.org>
Signed-off-by: NRob Herring <robh@kernel.org>