Crypto: Use RSA key in PSA format

Support for parsing DER format in the PSA Crypto API is
going to be removed from Mbed TLS 3.6.0, so just use the
PSA format directly in any RSA test that uses keys.

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: Iee602313c7e63ee663ac38fdef22374575e73af7