hwrng: npcm - Add struct device pointer to device context struct

Add a struct device pointer field to the device's context struct. This
makes using the unsigned long priv pointer in struct hwrng unnecessary, so
remove that one as well.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>