Documentation: bindings: Add support for Amlogic GXBB SCPI protocol

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
(decoupled from the generic scpi binding)
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>