drivers: bluetooth: Introduce SiWx91x HCI driver

Driver was tested with a custom application which enabled the BT_SHELL.
Basic functionalities were verified:
 - Scanning
 - Advertising
 - Connecting

Configuration needed for the test:
 - CONFIG_BT=y
 - CONFIG_BT_PERIPHERAL=y
 - CONFIG_BT_CENTRAL=y
 - CONFIG_BT_SHELL=y
 - CONFIG_SHELL=y

Co-authored-by: Tibor Laczko <tibor.laczko@silabs.com>
Signed-off-by: Tibor Laczko <tibor.laczko@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>