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:
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.