dt-bindings: Add vendor prefix for ilitek

This adds the vendor prefix ilitek for ILI Technology Corporation (ILITEK).

This prefix is already used several places and I will be adding more.

Signed-off-by: David Lechner <david@lechnology.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1511122328-31133-2-git-send-email-david@lechnology.com