dt-bindings: bus: Convert GISB arbiter to YAML

Convert the Broadcom STB GISB bus arbiter to YAML to help with
validation.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211217042001.479577-6-f.fainelli@gmail.com