Documentation: brcmstb-l2: Add Broadcom STB Level-2 interrupt controller binding

This patch adds the Device Tree binding document for the Broadcom
Set-top-box Level 2 interrupt controller hardware.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Link: https://lkml.kernel.org/r/1400892054-24457-3-git-send-email-f.fainelli@gmail.com
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>