crypto: atmel-ecc - factor out code that can be shared

In preparation of adding support for the random number generator in
Atmel atsha204a devices, refactor the existing atmel-ecc driver (which
drives hardware that is closely related) so we can share the basic
I2C and command queuing routines.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>