bt: hci_ecc: add option to use PSA APIs instead of TinyCrypt

This commit adds CONFIG_BT_USE_PSA_API to allow the end
user to prefer PSA APIs over TinyCrypt for crypto operations
in bluetooth. Of course, this is possible only if
a PSA provider is available on the system, i.e.
CONFIG_PSA_CRYPTO_CLIENT is set.

This commit also extends
tests/bluetooth/mesh/basic/bluetooth.mesh.gatt adding a specific
case using PSA.

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