dt-bindings: mailbox: qcom-ipcc: Add sc8280xp compatible

Document the sc8280xp compatible, and at the same time also make sure
the list is sorted alphabetically.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>