powerpc/xmon: Reset RCU and soft lockup watchdogs

I'm seeing RCU warnings when exiting xmon. xmon resets the NMI
watchdog, but does nothing with the RCU stall or soft lockup
watchdogs. Add a helper function that handles all three.

Signed-off-by: Anton Blanchard <anton@ozlabs.org>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200630100218.62a3c3fb@kryten.localdomain