dt-bindings: use isil prefix for Intersil in I2C trivial-devices.txt

This patch fixes I2C trivial-devices.txt DT documentation file 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: Wolfram Sang <wsa@the-dreams.de>