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 <[email protected]>
Cc: Peter Maydell <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Jason A. Donenfeld <[email protected]>
Signed-off-by: Stafford Horne <[email protected]>

Author: zx2c4

arch/openrisc/kernel/process.c

@@ -52,6 +52,8 @@ void machine_restart(char *cmd)
 {
 	do_kernel_restart(cmd);
 
+	__asm__("l.nop 13");
+
 	/* Give a grace period for failure to restart of 1s */
 	mdelay(1000);