mips: loongson64: remove unreachable(), fix loongson_poweroff().

On my Yeeloong 8089, I noticed the machine fails to shutdown
properly, and often, the function mach_prepare_reboot() is
unexpectedly executed, thus the machine reboots instead. A
wait loop is needed to ensure the system is in a well-defined
state before going down.

In commit 997e93d4df ("MIPS: Hang more efficiently on
halt/powerdown/restart"), a general superset of the wait loop for all
platforms is already provided, so we don't need to implement our own.

This commit simply removes the unreachable() compiler marco after
mach_prepare_reboot(), thus allowing the execution of machine_hang().
My test shows that the machine is now able to shutdown successfully.

Please note that there are two different bugs preventing the machine
from shutting down, another work-in-progress commit is needed to
fix a lockup in cpufreq / i8259 driver, please read Reference, this
commit does not fix that bug.

Reference: https://lkml.org/lkml/2019/2/5/908
Signed-off-by: Yifeng Li <tomli@tomli.me>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: stable@vger.kernel.org # v4.17+

arch/mips/loongson64/common/reset.c

@@ -59,7 +59,12 @@ static void loongson_poweroff(void)
 {
 #ifndef CONFIG_LEFI_FIRMWARE_INTERFACE
 	mach_prepare_shutdown();
-	unreachable();
+
+	/*
+	 * It needs a wait loop here, but mips/kernel/reset.c already calls
+	 * a generic delay loop, machine_hang(), so simply return.
+	 */
+	return;
 #else
 	void (*fw_poweroff)(void) = (void *)loongson_sysconf.poweroff_addr;