rv/rvgen: make monitor arguments required in rvgen

rv/rvgen: make monitor arguments required in rvgen

Add required=True to the monitor subcommand arguments for class, spec,
and monitor_type in rvgen. These arguments are essential for monitor
generation and attempting to run without them would cause AttributeError
exceptions later in the code when the script tries to access them.

Making these arguments explicitly required provides clearer error
messages to users at parse time rather than cryptic exceptions during
execution. This improves the user experience by catching missing
arguments early with helpful usage information.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/r/20260223162407.147003-15-wander@redhat.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>