Documentation: mailbox: Add APM X-Gene SLIMpro mailbox dts documentation

This adds the APM X-Gene SLIMpro mailbox device tree
node documentation.

Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Duc Dang <dhdang@apm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>