crypto: atmel-sha - create function to get an Atmel SHA device

This is a transitional patch: it creates the atmel_sha_find_dev() function,
which will be used in further patches to share the source code responsible
for finding a Atmel SHA device.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>