crypto: ccree - fix some reported cipher block sizes

OFB and CTR modes block sizes were wrongfully reported as
the underlying block sizes. Fix it to 1 bytes as they
turn the block ciphers into stream ciphers.

Also document why our XTS differes from the generic
implementation.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>