drivers: lora: sx1276: support TCXO power control

Add support for TCXO power control using GPIO pin. Some boards
(including B-L072Z-LRWAN1 already supported in Zephyr) need delay
applied after powering on TCXO, so add device-tree property allowing to
configure that as well. Cache information about TCXO power status, so
subsequent requests to enable it will not result in unnecessary delays.

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