psa: Fix slot number key attribute
psa: Fix slot number key attribute

Slot number key attribute is specific of dynamically
registered drivers and should thus not be computed/
returned in case of keys associated to drivers
implementing the new unified driver interface.

Signed-off-by: Ronald Cron <b35ff5b7ca26c4ae9b66b92f2cf34224c8e26b08@arm.com>

Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls