net: thunderx: add mutex to protect mailbox from concurrent calls for same VF

In some cases it could happen that nicvf_send_msg_to_pf() could be called
concurrently for the same NIC VF, and thus re-writing mailbox contents and
breaking messaging sequence with PF by re-writing NICVF data.

This commit is to implement mutex for NICVF to protect mailbox registers
and NICVF messaging control data from concurrent access.

Signed-off-by: Vadim Lomovtsev <vlomovtsev@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>