drivers: lora: Factor out sx12xx common functionality

LoRa radios supported by LoRaMAC-Node have a lot of common
functionality. Zephyr's LoRa implementation for the SX1276 uses
LoRaMAC-Nodes Radio HAL to implement API functionality like send and
recv. The exact same functionality will be used by the SX126x
driver. Facilitate sharing by moving that to a separate source file.

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