mbedtls: add Kconfig to select the number of key slot in PSA Crypto core

Adding new CONFIG_MBEDTLS_PSA_KEY_SLOT_COUNT to select the number
of key slots in PSA Crypto core. The default value is 16. Be aware
that key slots consume RAM memory even if unused, so the proper value
should be a compromise between the number of slots required by
the application and the available RAM in the system.

This commit also:
- updates tests/crypto/secp256r1/mbedtls.conf to showcase how to
  use this new symbol to reduce RAM footprint.
- tests/bsim/bluetooth/mesh/overlay_psa.conf to support all the
  keys used in the test.

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