dt-bindings: Document brcm, int-fwd-mask property for bcm7038-l1-intc

Indicate that the brcm,int-fwd-mask property is optional and can be set
on platforms which require to leave specific interrupts unmanaged by
Linux and need to retain the firmware configuration.

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/20191024201415.23454-5-f.fainelli@gmail.com