doc: bindings: mfd: act8945a: Update the example

Since the act8945a-charger is regarded as a sub-device and it using
"interrupts" property, update the examples section.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>