dt-bindings: mailbox: sti-mailbox: convert to DT schema

Convert the sti-mailbox.txt file into st,sti-mailbox.yaml

Signed-off-by: Alain Volmat <avolmat@me.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>