secure_storage: enable CONFIG_ENTROPY_GENERATOR in samples/tests

Explicitly enable CONFIG_ENTROPY_GENERATOR instead of relying on the
build system's defaults.

This:
- Makes sure the filtering works properly between entropy_driver and
entropy_not_secure test scenarios for the samples.
- Helps with TF-M builds in certain scenarios where key generation (via
`psa_generate_key()`) would fail due to the RNG functionality being
disabled.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>