dt-bindings: mailbox: imx-mu: add i.MX93 MU

Add bindings for i.MX93 MU which derived from i.MX8ULP

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>