hwrng: drivers - Use device-managed registration API

Use devm_hwrng_register to simplify the implementation.
Manual unregistration and some remove functions can be
removed now.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Acked-by: Łukasz Stelmach <l.stelmach@samsung.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>