Add ECDH support for ATECC508A (I2C) device.

The device features hardware acceleration for the NIST standard
P256 prime curve and supports the complete key life cycle from
private key generation to ECDH key agreement.

Random private key generation is supported internally within
the device to ensure that the private key can never be known
outside of the device. If the user wants to use its own private
keys, the driver will fallback to the ecdh software implementation.
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>