irqchip/brcmstb-l2: Add support for the BCM7271 L2 controller

Add the initialization of the generic irq chip for the BCM7271 L2
interrupt controller.  This controller only supports level
interrupts and uses the "brcm,bcm7271-l2-intc" compatibility
string.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>