crypto: atmel-sha - fix context switches

This patch saves the value of the internal hash register at the end of an
'update' operation then restores this value before starting the next
'update'. This way the driver can now properly handle context switches.

WARNING: only hardware versions from sama5d4x and later provide the
needed interface to update the internal hash value. Especially, sama5d3x
cannot implement this feature so context switches are still broken.

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