crypto: octeontx2 - add synchronization between mailbox accesses

Since there are two workqueues implemented in CPTPF driver - one
for handling mailbox requests from VFs and another for handling FLR.
In both cases PF driver will forward the request to AF driver by
writing to mailbox memory. A race condition may arise if two
simultaneous requests are written to mailbox memory. Introducing
locking mechanism to maintain synchronization between multiple
mailbox accesses.

Signed-off-by: Harman Kalra <hkalra@marvell.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>