mbedtls: fix entropy module enablement

- Do not set CONFIG_MBEDTLS_ZEPHYR_ENTROPY in
  tests/crypto/mbedtls because this can cause test failure on
  real devices in which test thread do not have access to
  drivers.
- make MBEDTLS_PSA_CRYPTO_RND_SOURCE depending on
  MBEDTLS_PSA_CRYPTO_C because it only makes sense when the
  latter is defined

Signed-off-by: Valerio Setti <vsetti@baylibre.com>