boards: nucleo_wb55rg: Add openocd support and set to default runner

Flashing stm32wb SoCs is available since Zephyr SDK-0.11.0.
Enables it on nuclmeo_wb55rg and set it as default as it is
faster in flashing and support doesn't require additional
action (vs pyocd package support).

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>