irqchip/brcmstb-l2: Set controller as wake-up source

Utilize the Broadcom interrupt controller standard property
"brcm,irq-can-wake" to flag whether this particular interrupt controller
instance is wake-up capable.

Since we do not know what type of parent interrupt controller we are
interfaced with, ensure that enable_irq_wake() is called early on.

Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200709223016.989-3-f.fainelli@gmail.com