drivers: lora: sx127x: rework SX1276 driver to add SX1272 support

SX1272 chip is very similar to the already supported SX1276;
unfortunately, Semtech provides a completely separate driver
for this one. This commit marks an attempt to avoid code
duplication by means of some not-very-pretty ifdefery.

Signed-off-by: Ilya Tagunov <tagunil@gmail.com>