crypto: octeontx2 - Add dependency on NET_VENDOR_MARVELL

The crypto octeontx2 driver depends on the mbox code in the network
tree.  It tries to select the MBOX Kconfig option but that option
itself depends on many other options which are not selected, e.g.,
CONFIG_NET_VENDOR_MARVELL.  It would be inappropriate to select them
all as randomly prompting the user for network options which would
oterhwise be disabled just because a crypto driver has been enabled
makes no sense.

This patch fixes this by adding a dependency on NET_VENDOR_MARVELL.
This makes the crypto driver invisible if the network option is off.

If the crypto driver must be visible even without the network stack
then the shared mbox code should be moved out of drivers/net.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 5e8ce833 ("crypto: marvell - add Marvell OcteonTX2 CPT...")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>