hwrng: imx-rngc - add quality to use it as kernel entropy pool

The RNGB can generate 2^20 words (1 word == 4 byte) of 'random' data
after the seed pool was initialized. The pool needs to be reseeded if
more words are required. The reseeding is done automatically since
commit 3acd9ea9

 ("hwrng: imx-rngc - use automatic seeding").

We can't retrieve the TRNG values directly so we need a other way to get
the quality level. We know that the PRNG uses 20.000 entropy samples
from the TRNG to generate 2^20 words (1MiB) and the quality level is
defined as (in bits of entropy per 1024 bits of input). So the quality
level can be calculated by:

   20.000 * 1024
   ------------- = ~ 19.5
        2^20

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>