hwrng: core - introduce rng_quality sysfs attribute

The rng_quality sysfs attribute returns the quality setting for the
currently active hw_random device, in entropy bits per 1024 bits of
input. Storing a value between 0 and 1024 to this file updates this
estimate accordingly.

Based on the updates to the quality setting, the rngd kernel thread
may be stopped (if no hw_random device is trusted to return entropy),
may be started (if the quality setting is increased from zero), or
may use a different hw_random source (if that has higher quality
output).

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>