dt-bindings: dsp: mediatek: Use meaningful names for mbox

Rename mbox according to actions instead of 'mbox0' and 'mbox1'.
The 8195 dsp node, which uses this binding, has not yet been added to
the 8195 devicetree.

Signed-off-by: Tinghan Shen <tinghan.shen@mediatek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220622062245.21021-2-tinghan.shen@mediatek.com


Signed-off-by: Mark Brown <broonie@kernel.org>