dt-bindings: media: gpio-ir-receiver: Document wakeup-souce property

The GPIO IR receiver can be used as a wake-up source for the system,
document that.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>