hwrng: optee - Use device-managed registration API

Use devm_hwrng_register to get rid of manual unregistration.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>