wireguard: notify key changes to crypto engine

This is a prerequisite patch for the following openssl API optimization
patch, which tries to offload openssl ctx init and key expansion work to
the initialization stage.

Wireguard adds crypto keys via vnet_crypto_key_add (), and whenever it
modifies the keys, the underneath openssl crypto engine shoud be informed
of the changes to update the openssl ctx.

Type: feature
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Change-Id: I3e8f033f3f77eebcecfbd06e8e3bbbfdc95a50e2