crypto: atmel-sha - Add zero length message digest support for hmac

Add softare padding to hmac-sha digest for zero length messages.
Using the atmel_sha_fill_padding() to fill the buffer with a padded
empty message with a length of the block size.

Create a temporary scatter list from the padded buffer to pass into the
data processing functions.

Signed-off-by: Ryan Wanner <Ryan.Wanner@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>