openrisc: define nop command for simulator reboot

The simulator defines `l.nop 1` for shutdown, but doesn't have anything
for reboot. Use 13 for this, which is currently unused, dubbed
`NOP_REBOOT`.

Cc: Stafford Horne <shorne@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Link: https://lore.kernel.org/all/YmnaDUpVI5ihgvg6@zx2c4.com/


Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>