rtc: isl1219: add device tree documentation

The devicetree documentation for the ISL1219 device tree
binding is added with a short example. It is not a trivial
device, because it supports two interrupt sources.

Signed-off-by: Denis Osterland <Denis.Osterland@diehl.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>