dt-bindings: firmware: convert meson_sm.txt to dt-schema

Convert the Amlogic Secure Monitor bindings to dt-schema.

Take in account usage the used variant with amlogic,meson-gx-sm.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221117-b4-amlogic-bindings-convert-v2-1-36ad050bb625@linaro.org


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>