rcutorture: Add NOCB02 config for nocb poll mode testing

rcutorture: Add NOCB02 config for nocb poll mode testing

Add new rcutorture config NOCB02 that enables rcu_nocb_poll boot
parameter combined with CONFIG_RCU_NOCB_CPU to exercise the polling
mode code paths in the NOCB implementation.

This config exercises poll-mode paths not covered by other configs,
where callback invocation uses active polling instead of kthread
wakeups.

This config is not added to CFLIST to avoid increasing the default
test duration; it can be run explicitly when poll-mode testing
is needed.

Acked-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>