bt: use PSA for HCI ECC and rename BT_TINYCRYPT_ECC

This commit sets PSA Crypto API as the default library to perform
ECC-DH in HCI, replacing TinyCrypt (which is being deprecated).
Therefore the symbol BT_TINYCRYPT_ECC is renamed as BT_SEND_ECC_EMULATION.
References in samples/tests are also fixed.

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