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 <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/interrupt-controller/brcm,bcm7038-l1-intc.txt

···
 - brcm,irq-can-wake: If present, this means the L1 controller can be used as a
   wakeup source for system suspend/resume.
 
+Optional properties:
+
+- brcm,int-fwd-mask: if present, a bit mask to indicate which interrupts
+  have already been configured by the firmware and should be left unmanaged.
+  This should have one 32-bit word per status/set/clear/mask group.
+
 If multiple reg ranges and interrupt-parent entries are present on an SMP
 system, the driver will allow IRQ SMP affinity to be set up through the
 /proc/irq/ interface.  In the simplest possible configuration, only one