drivers: lora: Add SX126x LoRa radio support

Add support for the SX126x series of LoRa radios using the
LoRaMAC-Node HAL.

This driver currently makes the following assumptions:

  * DIO1 is used as an interrupt line.

  * There is an RF switch selecting between the TX and RX ports and
    that switch is controlled by DIO2.

  * There is either no TCXO or the TCXO is controlled by DIO3.

Specifically, the limitations above mean that modules that use GPIOs
to control the RF switch are currently not supported. Support for such
modules would need changes to the LoRaMAC-Node code.

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