lib/crypto: sha512: Add support for SHA-384 and SHA-512

Add basic support for SHA-384 and SHA-512 to lib/crypto/.

Various in-kernel users will be able to use this instead of the
old-school crypto API, which is harder to use and has more overhead.

The basic support added by this commit consists of the API and its
documentation, backed by a C implementation of the algorithms.
sha512_block_generic() is derived from crypto/sha512_generic.c.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630160320.2888-3-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>