drivers: lora: Prepare Kconfig for additional radios

Add a new configuration option, LORA_SX12XX, that is shared for all of
the LoRaMAC-node-based radio drivers. By default, the appropriate
driver for the LoRa radio in the device tree is included is included
in the build.

Signed-off-by: Andreas Sandberg <andreas@sandberg.pp.se>