doc: Flash the S32Z from the CLI instead of an IDE

Leverage the improved support for the S32 development tools in Zephyr to
avoid instructing the user to launch the NXP IDE to flash the
development board. Additionally, remove the manual workaround step that
required the user to modify the value of a system register.

The instructions don't make use of the integration of the debug probe
with the west tool in order to avoid having Zephyr dependencies on the
machine that flashes the board.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>