dt-bindings: firmware: arm,scpi: Convert to json schema

Convert the old text format binding for System Control and Power Interface
(SCPI) Message Protocol into the new and shiny YAML format.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20210604205710.1944363-3-sudeep.holla@arm.com


[robh: Move fixed strings to 'properties', drop some literal block notations]
Signed-off-by: Rob Herring <robh@kernel.org>