dt-bindings: interrupt-controller: Document Broadcom STB HIF L2

Add documentation for the brcm,hif-spi-l2-intc compatible string to the
brcm,l2-intc.txt binding document.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200709223016.989-4-f.fainelli@gmail.com