crypto: arm64/sha2 - add generated .S files to .gitignore

Add the files that are generated by the recently merged OpenSSL
SHA-256/512 implementation to .gitignore so Git disregards them
when showing untracked files.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>