hwrng: octeon - Use devm_hwrng_register

Use resource managed function devm_hwrng_register instead of
hwrng_register to make the error-path simpler. Also, remove
octeon_rng_remove as it is now redundant.

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>