dt-bindings: add amc6821, isl1208 trivial bindings

TI AMC6821 fan controller and Intersil ISL1208 are trivial
devices, so add them to the binding list.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Signed-off-by: Rob Herring <robh@kernel.org>