mailbox: Add support for APM X-Gene platform mailbox driver

X-Gene mailbox controller provides 8 mailbox channels, with
each channel has a dedicated interrupt line.

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>