crypto: atmel-aes - switch to skcipher API

Commit 7a7ffe65 ("crypto: skcipher - Add top-level skcipher interface")
dated 20 august 2015 introduced the new skcipher API which is supposed to
replace both blkcipher and ablkcipher. While all consumers of the API have
been converted long ago, some producers of the ablkcipher remain, forcing
us to keep the ablkcipher support routines alive, along with the matching
code to expose [a]blkciphers via the skcipher API.

So switch this driver to the skcipher API, allowing us to finally drop the
ablkcipher code in the near future.

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Tested-by: NTudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: NArd Biesheuvel <ardb@kernel.org>
Reviewed-by: NTudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>