drivers: lora: sx1276: support antenna power enable

RF SPDT switches used for RX/TX selection have often power enable pin
connected to MCU's GPIO, so it is possible to disable it to save power
when not doing any data transfers. Add 'antenna-enable-gpios' property
to support such designs.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>