drivers: bluetooth: hci: Add BlueNRG reset API to ST HCI SPI BT driver

Add API to perform hardware reset optionally entering firmware updater
mode.

Remove redundant declaration for bt_spi_send_aci_config; otherwise, we will
have compiler warning if CONFIG_BT_BLUENRG_ACI is not set.

Signed-off-by: Ali Hozhabri <ali.hozhabri@st.com>