hwrng: npcm - Add NPCM8XX support

Adding RNG NPCM8XX support to NPCM RNG driver.
RNG NPCM8XX uses a different clock prescaler.

As part of adding NPCM8XX support:
- Add NPCM8XX specific compatible string.
- Add data to handle architecture specific clock prescaler.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>