dt-binding: hwrng: add NPCM RNG documentation

Added device tree binding documentation for Nuvoton BMC
NPCM Random Number Generator (RNG).

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