crypto: caam - pass key buffers with typesafe pointers

The 'key' field is defined as a 'u64' and used for two different
pieces of information: either to store a pointer or a dma_addr_t.
The former leads to a build error on 32-bit machines:

drivers/crypto/caam/caamalg_desc.c: In function 'cnstr_shdsc_aead_null_encap':
drivers/crypto/caam/caamalg_desc.c:67:27: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
drivers/crypto/caam/caamalg_desc.c: In function 'cnstr_shdsc_aead_null_decap':
drivers/crypto/caam/caamalg_desc.c:143:27: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]

Using a union to provide correct types gets rid of the warnings
and as well as a couple of redundant casts.

Fixes: db57656b

 ("crypto: caam - group algorithm related params")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>