drivers: bluetooth: hci_spi: Use DT instead of Kconfig to get HW params

Convert the HCI SPI driver to get the SPI and GPIO settings from Device
Tree instead of Kconfig. The "zephyr,bt-hci-spi" binding is used as
a common one for this purpose ("st,spbtle-rf" is removed), to take
advantage of the new DT_<COMPAT>_<INSTANCE> generated macros and get
rid of related fixups and aliases.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>