boards: raspberrypi: rpi_pico2: Add OpenOCD runner configuration

Add OpenOCD debugger support.
For now we will need Raspberry Pi'a forked version of OpenOCD from
https://github.com/raspberrypi/openocd

 .

The default adapter speed is set to match Raspberry Pi's documentation.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>