Documentation: bcm7120-l2: Add Broadcom BCM7120-style L2 binding

This patch adds the Device Tree binding document for the Broadcom
BCM7120-style Set-top-box Level 2 interrupt controller hardware.

We provide some ascii art explaining the different roles and reasons why
some properties are introduced to represent that interrupt controller.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lkml.kernel.org/r/1410309862-27784-3-git-send-email-f.fainelli@gmail.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>