hwrng: imx-rngc - use automatic seeding

The rngc requires a new seed for its prng after generating 2^20 160-bit
words of random data. At the moment, we seed the prng only once during
initalisation.

Set the rngc to auto seed mode so that it kicks off the internal
reseeding operation when a new seed is required.

Keep the manual calculation of the initial seed when the device is
probed and switch to automatic seeding afterwards.

Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>