fix: backport SX1262 PHY optimisations for RAK4631 from Meshtastic

[drupol]

This PR addresses several physical layer misconfigurations for the SX1262 transceiver on the RAK4631 (drawing upon proven optimisations from the Meshtastic project and RadioLib):

• TCXO Voltage: Corrects the DIO3 TCXO voltage from 3.3V to 1.8V for the RAK4631, aligning with the hardware specification to prevent frequency drift and potential component degradation.
• RTC Stop Errata (Semtech 15.3): Explicitly writes 0x00 to register 0x0902 during initialisation. This prevents the internal RTC from triggering spontaneous wakeups, avoiding silent hardware lockups.
• OCP Tuning: Increases the Over-Current Protection (OCP) limit to 140mA (0x38) for both RAK4631 and RAK3401.

This ensures the 22dBm transmit power is not artificially clipped by the default 100mA limiter, maximising range and signal integrity.