lib/crypto: sha512: Add HMAC-SHA384 and HMAC-SHA512 support

Since HMAC support is commonly needed and is fairly simple, include it
as a first-class citizen of the SHA-512 library.

The API supports both incremental and one-shot computation, and either
preparing the key ahead of time or just using a raw key.  The
implementation is much more streamlined than crypto/hmac.c.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250630160320.2888-4-ebiggers@kernel.org


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