irqchip/irq-bcm7038-l1: Support brcm,int-fwd-mask

On some specific chips like 7211 we need to leave some interrupts
untouched/forwarded to the VPU which is another agent in the system
making use of that interrupt controller hardware (goes to both ARM GIC
and VPU L1 interrupt controller). Make that possible by using the
existing brcm,int-fwd-mask property and take necessary actions to avoid
masking that interrupt as well as not allowing Linux to map them.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20191024201415.23454-6-f.fainelli@gmail.com