drivers: lora: Move board support to a separate file

The implementation of the board support routines is shared between all
LoRa drivers that use LoRaMAC-Node. Move them from the SX1276
implementation to a separate source file to facilitate reuse. Make
this source file conditional on CONFIG_HAS_SEMTECH_RADIO_DRIVERS since
it will be used by all LoRaMAC-Node-based drivers.

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