boards: nucleo_wl55jc: Fix openocd configuration

'connect_assert_srst' is preventing openocd to
connect to the board in debug mode.

To enable debug, fix openocd configuration to propose
two configurations:
- Debug compatible (default)
- Sleep mode compatible (commented)

Sleep mode compatible configuration should be used
when flashing over board's stock firmware.
A note has been added in board documentation to detail
this.

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