irqchip/brcmstb-l2: Match UPG_AUX_AON_INTR2 compatible

The UPG_AUX_AON_INTR2 Level 2 interrupt controller node is defined with
the "brcm,upg-aux-aon-l2-intc" compatible string in Device Tree and
behaves as an edge triggered standard Broadcom STB L2 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/20200709223016.989-7-f.fainelli@gmail.com