crypto: caam - fix RNG4 instantiation

The RNG4 block in CAAM needs to be 'seeded' first before being used
for generating pseudo-random data. The 'seeding' is done by getting
entropy from the TRNG ring oscillator. The RTFRQMAX register controls
the maximum allowable number of samples that can be aquired during
an entropy sample. Depending on the clock at which the RNG4 block
(and for that matter the SEC block) runs, it's possible that a
hard-coded value for the maximum frequency is inadequate, i.e. more
samples than needed are taken. This is an error, and thus the RNG4
block doesn't get initialized.  The patch attempts to alleviate
this issue by trying with progressivly larger frequencies, until
the number of samples is adequate.
This patch also fixes how a descriptor is deemed as being finished:
instead of checking the VALID field in the DECO debug register,
it makes sure that the DECO is idle, by checking the DECO state field
of the said register.

Signed-off-by: Alex Porosanu <alexandru.porosanu@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>