This is possible because after #8740 RSA_C no longer depends on
PK to parse and write private/public keys.
This commit also solves related issues that arose after this change
in "pk.c" and "test_suite_pk". In particular now we can use
rsa's module functions for parsing and writing keys without need
to rely on pk_parse and pk_write functions.
Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
