crypto: atmel-sha - add atmel_sha_wait_for_data_ready()

This patch simply defines a helper function to test the 'Data Ready' flag
of the Status Register. It also gives a chance for the crypto request to
be processed synchronously if this 'Data Ready' flag is already set when
polling the Status Register. Indeed, running synchronously avoid the
latency of the 'Data Ready' interrupt.

When the 'Data Ready' flag has not been set yet, we enable the associated
interrupt and resume processing the crypto request asynchronously from the
'done' task just as before.

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