hwrng: npcm - add NPCM RNG driver

Add Nuvoton NPCM BMC Random Number Generator(RNG) driver.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>