dt-bindings: Document brcm, irq-can-wake for brcm, bcm7038-l1-intc.txt

The BCM7038 L1 interrupt controller can be used as a wake-up interrupt
controller on MIPS and ARM-based systems, document the brcm,irq-can-wake
which has been "standardized" across Broadcom interrupt controllers.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191024201415.23454-3-f.fainelli@gmail.com