dt-bindings: PCI: brcmstb: compatible is required

The compatible property is required, make sure the binding documents it
as such.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211202223609.1171452-1-f.fainelli@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>