irqchip: bcm7120-l2: Add Broadcom BCM7120-style Level 2 interrupt controller

This patch adds support for the Level-2 interrupt controller
hardware found in Broadcom Set Top Box System-on-a-Chip devices. This
interrupt controller is implemented using a single enable register.

This interrupt controller is always present on the platforms supported
by the irq-brcmstb-l2 driver, hence the reason why both are compiled
using the same Kconfig symbol.

[jac] removed the following warning:
  drivers/irqchip/irq-bcm7120-l2.c: In function 'bcm7120_l2_intc_irq_handle':
  drivers/irqchip/irq-bcm7120-l2.c:49:27: warning: unused variable 'gc' [-Wunused-variable]

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lkml.kernel.org/r/1410309862-27784-2-git-send-email-f.fainelli@gmail.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>