ARM: dts: Enable Broadcom iProc mailbox controller

Reviewed-by: Jonathan Richardson <jonathan.richardson@broadcom.com>
Reviewed-by: Shreesha Rajashekar <shreesha.rajashekar@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Tested-by: Jonathan Richardson <jonathan.richardson@broadcom.com>
Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Jonathan Richardson <jonathan.richardson@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>