event/octeontx: add mailbox support

event/octeontx: add mailbox support

ssovf VF device has mailbox mechanism to communicate
with PF device. This patch adds support for a mbox API to
send the mailbox request to PF device.
The ssovf VF device will be used as the communication channel
to talk to PF devices of all the network accelerated
co-processors in Octeontx. Exposing as shared function to
use it from pool, crypto, network devices.

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Signed-off-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>