ARM: dts: brcmstb: add IRQ0 controller

This L2 controller handles multiplexing a few different interrupts. We
also need it for configuring the interrupt forwarding masks for the
UART.

With this, we can *now* boot BCM7445 to a prompt using the upstream
kernel + DTB.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>