crypto: nx - Constify static attribute_group structs

The only usage of these is to pass their address to
sysfs_{create,remove}_group(), which takes pointers to const struct
attribute_group. Make them const to allow the compiler to put them in
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>