crypto: arm/sha512 - Make sha512_arm_final static

Fix sparse warning:

arch/arm/crypto/sha512-glue.c:40:5: warning:
 symbol 'sha512_arm_final' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>