crypto: caam - make aamalg_desc a proper module

The drivers/crypto/caam/ directory is entered during build only
for building modules when CONFIG_CRYPTO_DEV_FSL_CAAM=m, but
CONFIG_CRYPTO_DEV_FSL_CAAM_CRYPTO_API_DESC is defined as a
'bool' symbol, meaning that caamalg_desc.c is always compiled
into built-in code, or not at all, leading to a link failure:

ERROR: "cnstr_shdsc_xts_ablkcipher_decap" [drivers/crypto/caam/caamalg.ko] undefined!
ERROR: "cnstr_shdsc_xts_ablkcipher_encap" [drivers/crypto/caam/caamalg.ko] undefined!
ERROR: "cnstr_shdsc_aead_givencap" [drivers/crypto/caam/caamalg.ko] undefined!
ERROR: "cnstr_shdsc_aead_decap" [drivers/crypto/caam/caamalg.ko] undefined!
ERROR: "cnstr_shdsc_aead_encap" [drivers/crypto/caam/caamalg.ko] undefined!
ERROR: "cnstr_shdsc_aead_null_decap" [drivers/crypto/caam/caamalg.ko] undefined!
ERROR: "cnstr_shdsc_aead_null_encap" [drivers/crypto/caam/caamalg.ko] undefined!
ERROR: "cnstr_shdsc_rfc4106_decap" [drivers/crypto/caam/caamalg.ko] undefined!
ERROR: "cnstr_shdsc_rfc4106_encap" [drivers/crypto/caam/caamalg.ko] undefined!
...

Making caamalg_desc itself a loadable module fixes this configuration
by ensuring the driver gets built. Aside from making the symbol
'tristate', I'm adding appropriate module metadata here.

Fixes: 8cea7b66

 ("crypto: caam - refactor encryption descriptors generation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>