msm: mdm: Add shutdown ioctl and send poweroff request to the mdm

When the whole phone is being powered off, a poweroff request needs
to be sent to the external modem so that it can shutdown gracefully.
This request needs to be triggered from userspace before kernel drivers
start unloading so that other drivers needed to send the request are
still available. The shutdown ioctl is provided for this purpose.
The request is sent over system monitor.

Crs-Fixed: 401598
Signed-off-by: Joel King <joelking@codeaurora.org>