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

arch/arm64/crypto/.gitignore

+sha256-core.S
+sha512-core.S