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>

drivers/crypto/caam/Kconfig

@@ -136,4 +136,4 @@ config CRYPTO_DEV_FSL_CAAM_DEBUG
 	  information in the CAAM driver.
 
 config CRYPTO_DEV_FSL_CAAM_CRYPTO_API_DESC
-	def_bool CRYPTO_DEV_FSL_CAAM_CRYPTO_API
+	def_tristate CRYPTO_DEV_FSL_CAAM_CRYPTO_API

drivers/crypto/caam/caamalg_desc.c

@@ -1300,3 +1300,7 @@ void cnstr_shdsc_xts_ablkcipher_decap(u32 * const desc, struct alginfo *cdata)
 #endif
 }
 EXPORT_SYMBOL(cnstr_shdsc_xts_ablkcipher_decap);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("FSL CAAM descriptor support");
+MODULE_AUTHOR("Freescale Semiconductor - NMG/STC");