lora: LoRa Basics Modem backend for SX12xx chips

As a first integration of the LoRa Basics Modem backend, implement the
LoRa API for the standard SX126x/SX127x chips.

Much of the logic from `lbm_common.c` is taken from the loramac-node
`sx12xx_common` implementation, but it should now be agnostic for all
LoRa RF transceivers.

Signed-off-by: Jordan Yates <jordan@embeint.com>