dt-bindings: use isil prefix for Intersil in vendor-prefixes.txt

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: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Rob Herring <robh@kernel.org>