arm64: dts: apm: mailbox device tree node for APM X-Gene platform.

Mailbox device tree node for APM X-Gene platform.

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