dt-bindings: interrupt-controller: Document UPG auxiliary L2

Define the compatible string brcm,upg-aux-aon-l2-intc which is used by
the Broadcom STB UPG auxiliary always-on interrupt controller.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200709223016.989-6-f.fainelli@gmail.com