boards: arm: rpi_pico: add openocd runner configuration

Add openocd configuration.

This configuration allows selecting what debug adapter debugging rpi_pico
by a command-line argument.
It is mainly intended to support both the 'picoprobe' and 'raspberrypi-swd'
adapter described in "Getting started with Raspberry Pi Pico".
And any other SWD debug adapter might also be usable with
this configuration.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>