boards: rak: Add Rak11720 board

Added support for the Rak11720 board. Rak11720 is a WisBlock Core module
with Apollo3 Blue SoC from Ambiq and a Semtech SX1262 LoRa® transceiver.
For more information about this board please check:
https://docs.rakwireless.com/Product-Categories/WisDuo/RAK11720-Module/Overview/#product-description



There are known issues that have been identified while communicating
with SX1262 transceiver:

- Gpio interrupt handling error: ambiq_gpio_pin_interrupt_configure
function is not able to configure SX1262 Dio1 interrupt pin correctly.
Please check issue 73958.
- Spi communication error: Rak11720 uses a custom CS pin.
Firstly, custom CS pin support is necessary. Secondly, spi_ambiq_xfer
function is not able to communicate with SX1262 transceiver.
Please check issue 73959.

These issues have been documented for future solutions. Despite these
limitations, the current support enables basic functionality.

Examples can be build by below command for Rak11720:
west build -b rak11720 samples/hello_world

Signed-off-by: Sercan Erat <sercanerat@gmail.com>