modules: mbedtls: Build psa_crypto_slot_management conditionally

TFM redefines functions from mbed TLS's psa_crypto_slot_management.c,
therefore that file should not be included in build when TFM is enabled.
Otherwise, the linker reports an error about redefined functions like
psa_open_key() etc.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>