crypto: atmel-aes - Allocate aes dev at tfm init time

Allocate the atmel_aes_dev data at tfm init time, and not for
each crypt request.
There's a single AES IP per SoC, clarify that in the code.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>