crypto: ecc - update ecc_gen_privkey for FIPS 186-5

FIPS 186-5 [1] was released approximately 1 year ago. The most
interesting change for ecc_gen_privkey is the removal of curves with
order < 224 bits. This is minimum is now checked in step 1. It is
unlikely that there is still any benefit in generating private keys for
curves with n < 224, as those curves provide less than 112 bits of
security strength and are therefore unsafe for any modern usage.

This patch also updates the documentation for __ecc_is_key_valid and
ecc_gen_privkey to clarify which FIPS 186-5 method is being used to
generate private keys. Previous documentation mentioned that "extra
random bits" was used. However, this did not match the code. Instead,
the code currently uses (and always has used) the "rejection sampling"
("testing candidates" in FIPS 186-4) method.

[1]: https://doi.org/10.6028/NIST.FIPS.186-5

Signed-off-by: Joachim Vandersmissen <git@jvdsn.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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